Incapacitated

An enemy cannibal is often incapacitated after several hits with a melee weapon. The cannibal is knocked to the ground, and appears to be dead. However, if the player looks closely and listens, the cannibal's chest moves as it breathes and it has hissing, gasped breaths. When incapacitated, the cannibal requires an extra hit in order to kill. If the player does not kill the cannibal, it will rise back up and begin to attack the player anew. For this reason, it is well advised to make sure enemies on the ground are dead with an extra hit before moving on to the next enemy, to prevent a surprise attack from behind.

This is a different mechanic from a cannibal simply being knocked down. This, like being knocked down, only happens to cannibals and not mutants.

Cannibal Behavior
Other cannibals will react differently to one of their own being incapacitated. Starving cannibals will attempt to eat the cannibal while it is still alive. Tribal cannibals, if not being engaged in direct combat, will attempt to drag their own to safety, but will ignore incapacitated starving cannibals.