Very simple and straightforward. Just cut off arms (as opposed to legs and heads, those don't count) in the testing chamber... mine unlocked halfway through episode 4.
If you're wondering where the testing chamber is, it is accessible from various locations - if you have finished the game, just start it on NG+ or NG++ or whatever and progress through the game a little bit. You need to start the generator in the dark area of the first location of the game first. The entrance is right there, at the beginning of the underground area (but the door will be inaccessible until you restore power).