Ah, yes! The hunter roll. It only works if the survivors hit the hunter from the "Left" hip. Anywhere else he will just stumble.
Taken from the L4D wiki: The Hunter's roll, a very rare occurrence in Left 4 Dead, has been made more common. If the Hunter is shoved off his victim in the left hip, he may perform a back roll and be ready to pounce again in about a second. The Hunter can perform this move in any and all game modes.