Sadism only increases if Arousal is > 80%. Biting and headbutting is most effective.Interesting.
Reading the code shows sadism improves damage output of defiant actions. A flat damage increase i think.
But i cant really figure a decent way to improve it. I have tried but had bad luck of unwillingly finishing too early from a touch or something, while also im unsure the stat increases properly ... like i improved masochism level to 1 by accident while having nearly no sadism. I fight out of every unwanted encounter so logically that makes no sense.
So, get yourself in the right mood and the right position before you hit.
If one bite is not enough you'll get some Masochism for free...