In order to break an enemies neck you need to get them in a choke hold.
In order to get an enemy into a choke hold you need to be stationary behind said enemy and press the square button. This can be tricky as, if you are not stationary you will not choke them, but rather throw them, and that is something you do not want to do.
Once you have an enemy in the choke, repeatedly press the square button until, well, you break his neck.
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