Backstabbing is the action of a fatal strike against an opponent from behind with a Sword or a Dagger, thereby killing them instantly. Zombies, however, will enter their dormant State instead. Its disadvantage is that the enemies may cry out as they die, alerting those nearby, and it tends to leave blood stains. It counts as a murder, which it almost always a fail on Expert.
In Thief I and IIEdit
In the first two games, it is not required to stand behind a target, it is only required that a target was unaware of Garrett's presence at the moment his attacks connect. Since hitting an unaware target gives a stealth bonus - multiples the damage by 5 - the damage of the stealthy overhead sword attack would be: 6 times 5 = 30, a quick slash: 2 times 5 = 10 (can be applied several times in quick succession before the target actually gets aware). It is an effective way of assassination and disposal of enemies. Even highly supernatural entities like the Fire Elementals are vulnerable. Some creatures (like the Apparition: 32 HP) have more HP than one stealthy overhead strike (30 dmg) can inflict, which require additional attacks to finish them off.
In Thief: Deadly ShadowsEdit
In the third game, Garrett uses a Dagger instead of a sword, and had an specific animation when executed, wherein he drives the point into his opponent's back with both hands. Mechanically, executing a proper backstab in this game requires Garrett to be behind an unaware enemy. When positioned properly, Garrett will raise his hand with a reverse gripped dagger ("ice-pick grip"). It is only then that a backstab can be initiated. This backstab kills all enemies in a single hit, even Tree Beasts (though undead instead go into a dormant state).