It's emotions and a bit of a mental breakthrough with the bondage thing, and it's also a lot of physical pain, not to mention the endorphins making everything ten times more intense. Jack holds him close, steady and patient, time flowing and ebbing as he waits, without any sign of wanting to hurry him. He really doesn't want to hurry him. This, for him, is what it's all about, really. The feeling of closeness from having shared something so intense, from having got through it together... that's what he does kink for. The sex really is secondary to all that. It's a useful way to release the tension - and, frequently, a mind-blowingly wonderful way to do it - but it's this closeness afterwards that really does it for him.