The backstab command is usable only by thieves and assassins, and allows them to do 2-3 times more damage than normal with sharp or thrusting weapons. The thief or assassin must be hidden in order for a backstab to work, and if he or she fails, then they are considered vulnerable and will have to wait 3 times the normal amount of time before attacking again. Backstab can only be used while hidden and you cannot hide when in front of an aggressive opponent. This means you need to flee, hide and then sneak back again if you want to risk another backstab.