A backstab is a context-sensitive attack that a Spy can perform with any of his melee weapons. When a Spy uses any one of his knives to hit an enemy from behind, a successful backstab is performed, resulting in an instant kill in almost all cases. Backstabs deal no damage to enemies with invulnerability effects, such as those under the effects of a standard ÜberCharge, though the Spy still raises his knife and performs the usual backstab animation.
Should a Spy perform a melee attack that connects with an enemy while he is behind said enemy, a backstab occurs. Backstabs deal damage equal to 200% of the target's current health and are always critical hits, resulting in a total damage of six times the victim's current health; while the critical damage can be negated, this does not stop the stab from being lethal unless the target has significant additional protection.
A backstab triggers as long as the Spy is anywhere within a 180° angle behind the enemy, utilizing the enemy's current aiming direction as straight forwards. To indicate that a Spy is within the required range and angle to perform a backstab, he raises his knife and turns it downward, primed for a kill. The knife remains in this position until the target is no longer in reach, which is 40 Hammer units to the target's back, or the Spy attempts a backstab by attacking. The knife turns downward even if positioned behind a cloaked or disguised enemy Spy. As the Spy's melee weapons do not have a delay between the swing being initiated and the damage being dealt, as long as the knife is pointed downward, the Spy simply needs to attack for an instant kill to occur.
Like normal melee attacks, backstabs can be performed while disguised, though the Spy proceeds to lose his disguise whether the attack connects or not. Backstabs are nothing more than pure damage, and so cannot injure or kill invulnerable players. Buildings and Halloween bosses cannot be backstabbed.
Robots in Mann vs. Machine can be backstabbed just as normal players. In addition to this, if a robot is sapped, it can be backstabbed from any direction, not just from behind. However, giant robots operate much differently. Backstabbing a giant has a base damage of 750, but this is reduced to 25% of that (188) by default; each armor penetration upgrade restores 25% of the base damage, up to 125% of the base (938). Spies must therefore upgrade their knife in order to have a chance at taking out giant robots.
Various weapons affect backstabs in some fashion:
- Backstabbing with Your Eternal Reward or the Wanga Prick causes silent death. The victim does not scream, their body vanishes, and no killfeed notification is given to the victim's teammates. The attacking Spy disguises as the fallen enemy almost instantly.
- Backstabbing with the Conniver's Kunai takes the victim's remaining health and gives it to the Spy, up to a maximum of 210. This is offset by a lowered base health.
- Backstabbing with the Big Earner fills the Spy's Cloak meter by 30%, regardless of which cloaking device is equipped. A backstab with this weapon also gives the Spy a three-second speed boost.
- Backstabbing with the Spy-cicle turns enemy ragdolls into an ice statue, which also creates a loud freezing/cracking noise.
- Attempting to backstab a Sniper wearing the Razorback results in the backstab being blocked, in addition to preventing the Spy from attacking, cloaking, or switching weapons for 2 seconds afterward. Thus, the Razorback makes Snipers less prone to backstabs and encourages Spies to use their selected revolver instead.
- Stab-and-Sap: Stab a (turtling) Engineer, before switching to your Sapper and sapping the Engineer’s Sentry Gun; this must be done fast enough that the Sentry Gun does not have time to turn around and kill you. An experienced Spy may track the Sentry Gun's turning direction and move against it after he stabs (as it follows him, this buys him more time), this set of movement is also known as circle-strafing. Using Your Eternal Reward can make the approach easier, as you instantly disguise upon a successful backstab, preventing the Sentry Gun from detecting you and retaliating; a Spy without a prior disguise must rely on cover to avoid the Sentry Gun's attention, as the Sentry is not fooled by your instant disguise once it has already begun firing. This tactic is slightly easier on Engineers using the Wrangler, because the Sentry Gun is disabled for three seconds after the Engineer's death.
- Sap-and-Stab: Sap an Engineer's unattended building as bait, hide, then stab him when he returns to remove the Sapper. Alternatively, sap a turtling Engineer's building, then switch to the Knife to backstab him as a less effective variant of the stab-and-sap technique.
- The Topstab: Sometimes paranoid Engineers place their Sentry Gun and Dispenser in a fashion that allows the Engineer to have his back to a corner, making him seemingly protected from backstabs. It is possible to kill him by climbing over his Dispenser or Sentry Gun and jumping onto his head. Face the same direction as the Engineer and crouch until you get the "Knife-raised" animation, allowing you to score a backstab.
- Quickstab: While an unaware enemy is running towards you, let them run past you before spinning and stabbing their retreating back. This is one of the only reliable methods of backstabbing Scouts or any enemy that believes your disguise.
- Sidestab: As the name suggests, a side stab is accomplished by stabbing the back of an enemy from the side. Because of how the characters' hitboxes are positioned, this is easier from the enemy's right than their left.
- Dropstab: This involves the Spy waiting at a high spot, and jumping down on an unsuspecting enemy's head while holding crouch, and killing the enemy in a similar fashion as the Topstab.
Trickstabs happen when an enemy has spotted the Spy, and the Spy tries to outmaneuver the enemy in order to attempt a backstab.
- Matador: Usually most effective against support classes attempting to engage in melee combat (including Engineers), this involves tricking an enemy into turning towards in your direction by pretending to run around them one way, then quickly moving back, strafing into backstabbing range as they turn toward, and backstabbing your pursuer.
- Stairstab: This technique involves running up a flight of stairs (or a ramp) and jumping over your pursuer's head, landing behind or on top of them and scoring a backstab. This is less effective against more experienced players, as it is often highly predictable and can be easily countered.
- Cornerstabs: This takes advantage of the map such that the enemy pursuing the Spy would turn a corner (which does not necessarily need to be blind), and exposing their side to the Spy, in a similar fashion to the Matador stab.
- Circlestrafe stab: A highly advanced yet unreliable type of trickstab that is designed to counter strafing enemies who do not turn enough to fall for a matador, or enemies who backpedal. There are currently four known types.
- For enemies who are holding W and a strafe key while strafing into the Spy, the Spy moves into and intersects the path of the pursuing enemy.
- For slow, backpedaling enemies (e.g. Soldiers and Demomen), the Spy backpedals along the enemy's side and attempts a sidestab.
- For enemies who are strafing between two points, such as a Scout aiming towards a Spy, the Spy strafes to the opposite direction of where the enemy is strafing, and relying on the enemy to strafe back into them (e.g. A Scout moves towards the Spy's left side, the Spy moves towards his right side, and eventually the Scout strafes back into the Spy while attempting to strafe-aim).
- For enemies who are attempting to strafe behind the Spy, the Spy matches the enemy's movement by holding S and a strafe key, and attempts a sidestab while backpedaling.
- Surfstab: An extremely situational stab where the Spy surfs explosive damage or enemy airblasts, and kills the/another enemy while landing on them, similar to a Dropstab.
- Airstab: Also an extremely situational stab where both the Spy and the enemy get near each other while airborne, the stab is applied specially at high speeds, similar to a sidestab, the Spy quickly flicks his aim to the enemy's back and then stabs. To prevent the loss of your momentum while airborne due to bumping, strafe around the enemy and then stab.
- Due to glitch in the game, a stab that has appeared to have occurred from the front can sometimes be rendered as a backstab. Known more commonly as a facestab, these kills are now mostly the result of lag compensation by the game engine itself. Prior to many game patches, facestabs were more common due to bugs in the way the game detects a backstab. Facestabs take two forms: the "valid" stab and the "true" facestab. Most of what the recipient sees as a facestab is usually either a cornerstab or a sidestab, where on the Spy's screen, he sees it as attacking the target's back, but due to lag, the target sees the Spy attacking from the front. A valid facestab works as intended by the developers, making correct use of lag compensation. In a true facestab, the Spy appears to be attacking an enemy from the front on both players' screens, but lag compensation results in it being registered as a backstab. This is considered a bug.
- Another community-coined term is the failstab, which refers to a backstab which should have succeeded, but instead did only normal Knife damage, despite the backstab animation playing fully and the Knife connecting. These stabs are almost always linked to the latency conditions between the server and the player. A similar variant is a stab that plays the animation and the sound effect, but does no actual damage.
- Rarely, during taunts, the player's 'back' changes position; it becomes directly behind the player's 3rd-person camera, which means that players can avoid backstabs by moving the camera away from the Spy. This generally occurs by accident; for example, when you position the camera in front of you to watch the taunt yourself, making your back into your front. Facestabs can also rarely occur this way.
- The angle required for a backstab is based on the direction the victim is looking; however, all players have models whose normal stance has their backs rotated to the left slightly, due to the way they carry their respective weapons (this is most prominent with Heavies). This means that at certain angles you appear to be stabbing your target in the back, when in fact you are not inside the 180 degree angle for backstabs. Equally, there are valid backstab positions which appear to have you stabbing slightly into the front of your victim. These hit-box positions can be determined by the shoulders of the character models.
- When a player for the opposing team is standing on top of the Spy, the Spy can always perform a successful backstab regardless as to whether or not the Spy's aim falls within the regular backstab radius.
- Occasionally, when swiping at an enemy, the Spy will do the backstab animation, even if it wasn't a backstab.
- Occasionally, a Spy will take fall damage after backstabbing someone while standing on top of him, even though this was supposedly fixed in October 18, 2011 Patch.
- Originally, the Spy's Backstab animation was just over a second long and manifested as the current animation, but was slowed down. This was changed to the current, sped up version used today in the interests of balance. Before this change, skilled Spies would "quick-stab". This involved staying out of Backstab range of the victim, and then swinging the Knife just before entering Backstab range, thereby bypassing the Backstab animation, but still rewarding an instant kill.
- The Sniper's unique backstab animation was first seen in Trailer 2 before any backstab animations were integrated into the game.