These zombies, found in The Zombie Island of Dr. Ned add-on, spawn without legs, however these little guys can be annoying as they are ever so slightly stronger than regular zombies and they are difficult to see in the middle of a battle. They have a single target ranged attack, like the defiler's.
Because they are even slower than regular zombies, they're often the last enemies you'll face from a given mob, which makes it harder to spot them due to all the re-killed bodies lying around. Throwing an explosive contact grenade at a crowd might result in them getting killed.
Torsos are so short that they can be completely submerged in bodies of water. A quick solution is to equip a shotgun, throw a Transfusiongrenade, follow the trails in the gunsight, and shoot the spot where they turn red and start returning, if the trails haven't already re-killed the torso.
They can be extremely easy to get a headshot on when they aren't in water or obstructed by cover.
There are a large amount of torsos at Shady Oaks in Hallow's End.
If regular zombies are shot in the legs, they will turn into torsos, but their name will not change. A common glitch that happens on the PS3 is the amputated zombie will be animated to be moving very fast, but will in actuality be moving slowly backwards.
Torsos can be surprisingly hard to hit being low to the ground.