So fucking hot. I think this happens from the subtle motion of the cock bouncing around + obviously the prostate getting slammed by the cock. The harder the top pounds equates to more bounce of the bottoms cock and more stimulation of the prostate.
I get off just like that ,I still remember my frist time I got off like that my best friends mom walked into his room and he had me bent over the couch and was blowing his huge load in my ass ,his load was running all out me and I started spewing my load all over the floor and couch as his mom froze and watch me coming hard along wotg her son ,whsy a ppwerful orgasm and wasnt even tputched so great I still take his huge cock today when we cwn sneek off alone