How do I rip off an enemy's arm in Stubbs the Zombie?
You have to melee the enemy enough to stun them (but most enemies will then just die).
From what I observed, this triggers only for strong enemies (like the chainsaw guys), or soldiers with a helmet, since you can't eat their brain.
The button is then the same as for eating brain.
I've only played the demo but the instructions are:
Approach the enemy and press the Y button to tear his arm off