My favorite position when fucking is face-to-face, with the bottom on his back. I find it more intimate and prefer having everything in such close proximity and CBT is easily facilitated (among other things).
It can range from pulling with progressive firmness on the guy's nutsack or foreskin to slapping. Not every guy enjoys this, and most every guy has pretty sharp limits with it, so you need to be especially cautious and vigilant in watching his reactions. The fine line between pleasure and pain is prosciutto-thin when it comes to CBT, and not every guy will volunteer liking it, so you've gotta go slow and push to the limit (but not beyond).
Guys who wear ball-stretchers aren't any more likely to enjoy CBT than anyone else, which I find odd but experience has shown to be true. Ball-stretchers would seem to invite extra attention, but in most cases the guys wearing them are pretty much at their limit right there.
In addition to pulling (occasionally very hard) on foreskins, I have met a couple of guys who like to have their dicks chewed (not bitten) and many who like having their hardons pinched or slapped: it's all part of rough play.
I, myself, have no interest in getting my balls hit, pinched, kicked or bitten, and my limit at pulling is pretty mild, though I understand the pleasure involved. Since my own tolerance for nipple-tugging, especially after a couple of hours, can reach extreme limits, too, I just see this as a natural progression from simpler passions to more complex ones and no less respectable or necessary.