He cannot use his hands to grab the top's hair or grope his back or any part of his body, therefore his frustration is out of control and he will do anything to cum and get his desired orgasm.

He gets to play with all he wants because the other literally has no control or power. He gets to fuck the tied one in the pace and rhythm he wants, he gets to control his pleasure-suspend his orgasms or impede it. You have full control this time so expect that inbound porn, the perspective is on the top of the dominant one. As soon as they are tied, that is when you get to fuck them from behind. Bound sex is basically a kind of bondage sex, where you get your partner tied (often from behind), as a prisoner in surrender or caught in the act.