Effect:
Backstabs immobilize both the spy and the victim (Spy's disguise is removed). The victim is set on fire, and takes 20% of their maximum hp in fire damage every .5 second. The victim is knockback immune, if the spy is knocked away before the victim dies, the spy takes damage equal to 50% damage dealt during the backstab attempt. After a successful kill, spy gains hp equal to 30% damage dealt and cloaks for free for 3 seconds while gaining a 30% movement speed and 20% jump height buff for 3 seconds. Decloaking from this state takes only .8 seconds.

+25 max hp
-Take crits when you would normally take mini-crits
-Cannot heal while cloaked or disguised
-Weapons cannot be switched while cloaked
-Targets that are healed while "The Effect" is active cannot die and are reduced to 1 hp. The effect then ends with no benefit to the spy. (Medic Regen does not count)

The idea is that you have to be careful about taking inadvertent damage while staying undercover. You can either take the chance that people might not notice you while you stab their comrade, or pick off a sniper or straggler, then chain kills using the resultant buff. Without a kill, it's a fun but sub-par weapon.