The skill increases the cock requirement for cum on her body/face. While it costs willpower each turn (so you'll want a decent mind stat to keep it up in battle) the effect scales with sadism.
If you want to stop cum in a legit way, get some sadism levels (which will most likely come automatically with that playstyle) and keep the cock desire low-ish. For example, with three sadism levels, you need to keep the cock desire below 135. Keep in mind that maintaining the skill needs
10
willpower each turn or it get's cancelled.