A painful play scene is a way of giving M's body stimulation or pain through whips, low-temperature candles, etc. Many people think that pain play is simply to give M pain, but in fact, this kind of play mixes pain and pleasure. Because all pain sensations cause the central nervous system to release endorphins—proteins that relieve pain, their mechanism is similar to that of narcotics such as morphine, which can produce pleasurable feelings.
So to be precise, restraint play combines pleasure and pain and makes the brain confuse pleasure and pain to bring about stronger pleasure. (Tips: Painful play involves a lot of risks, please pay attention to safety.)