While I love a guy who is actively masochistic and directly gets off on pain, as most people know I much prefer pushing someone well past their threshold of enjoyment. This doesn't necessarily have anything to do with the intensity of what is being administered, but rather how it is impacting the boy involved. I want a boy to know I'm in his head, and that it's not only his body I'm in control of. It may sound melodramatic, but it really amounts to a battle of wills.
If you've ever stubbed your toe so bad you yelled or cursed, it should be easy to understand that this is your body/mind's attempt to process the pain. Naturally, people have different coping mechanisms and they can vary even for an individual through different levels of pain or discomfort. Sometimes this might be as significant as their demeanor completely changing, or as subtle as clenching their jaw. If you're not very observant on an intuitive level, well-paced repetition is the best way to glean new information - too frequent of repetition can end up with lingering responses compounding instead of giving information on a singular stimulus.
Once you've got a feel for how a boy is responding to a number of different stimuli, you want to deliberately elicit those reactions. What ends up happening is the more you force someone to use these coping mechanisms, the more they become dependent on them. Essentially, it almost works like addiction: the more you make a someone rely on a coping mechanism, the more dependent on it they become. The result is it takes an increasingly small amount of effort to evoke a response. This doesn't necessarily have to be done by intensifying the specific action, it's just as useful to simply reduce the cooldown before you start up again. The latter is a much safer alternative given that, for someone with a high pain tolerance to begin with, you run the risk of escalating to the point of causing harm.
Now that you've got a guy flinching in anticipation and reacting when you barely touch him, you have a lot more flexibility with how far you can choose to take a guy. To quantify it, eventually you could get a guy to react as severely at a 3/10 intensity level as he used to at 7/10. What this means is that you have a lot more room to take a guy's brain further before you cap out at what is physically safe. Suddenly a guy doesn't freak out at 9/10 intensity and then you have no place else to go, leaving you incapable of correcting the behavior. This is where training opportunities begin.
The goal with any training should be to make as staunch of a difference as possible between desirable behavior and unwanted behavior. Use this newfound, elevated dependence on coping mechanisms to coax out whatever reaction you're trying to quell, and increase the intensity drastically as soon as it presents. Since by this point a guy's head will be pretty fuzzy, he may need a verbal reminder of why the intensity is ramping up; his brain can be so overwhelmed it's letting his body operate unintentionally.
At this point it's really as methodical as lather, rinse, repeat; the more frequent the reaction you're trying to stop arises, the greater the gap in intensity should become. If you can isolate enough individual behaviors this way, you're very apt to have a boy that just whimpers pathetically and becomes putty in your hands - not to mention one whose brain is frantically trying to predict what comes next.