This is a game of subtleties you don't need to start off hard and fast. With CP you need to build up with other stimulation the same is true, getting him good and horny. You need to know about all the things he likes, and what is on the edge of his fantasies. For example you can use verbal to reinforce his identity, you can talk about torture or humiliation scenes he wanks about but is too scared yet to go there. You create images for him making them are colorful and real in his imagination as possible, like the imagination does the hard work (see hypnosis). Whatever you do you are giving 90% to 110% pushing at the limits and easing off to let him catch his breath.
An experienced Master might start with just a small amount of pain, known as a "warm up". This will cause endorphins to be released. This will enable the masochist to tolerate more pain. Gradually, the sadist will increase the amount of pain, which will in turn increase the amount of endorphin. If the sadist has a sufficient amount of patience and skill, he or she can enable the masochist to experience tremendous euphoria, and the amount of pain the masochist actually experiences may be relatively small.
The slaves facing the potential danger of the activity is part of the thrill, and the fear pushes them to meet the challenge of reaching the top of this peak. Similarly, some people experience S/M as a way of "pushing the envelope" and forcing themselves to break through barriers, and they love the rush of doing something risky and exciting.
There is one huge rush with the feeling of "Oh Fuck!!!"
Just as there is with solo play, where just pretending is no longer good enough.
Just like they need for some slaves to play with KNOWN 'dangerous' Tops, its the next step, as nice concenting tops will stop too quickly. On the other hand compare that to a Master/Sadist that will do what the fuck he likes and only stops when that slave has shot his filthy load onto the floor and had his faced pushed into the dirt to lick it up.
One measure is how quickly the slave comes back?
This is usually part of a memory fade process. The slave initially might say never again (maybe bad mouth the "dangerous/psychopath" Master) and then he will have the urge aghain, as nothing else carries the adrenaline pump that you get in some scenes.
This is different to the just keep me as a xxx slave.
If you need the rush, not much else will do.