I have a long term buddy that really enjoys FF. For the most part, he likes to take the top role when we play. If we play at my house, I lie back in the sling, hole right at the edge and because we have played for years, I trust him. That is an important part of making the experience enjoyable for me. We all know that sometimes the real pig comes out and you are open to almost anything. Other times, your hole might be tender or your ass just isn't in a stretching mode, despite some 420 and popper help. The trust part is that he can FEEL whether I am relaxed and at times, he pushes me over the edge a little when I am teetering... and the magic starts. He knows enough about me and my ass that he is happy to take control of my hole and typically starts with a solid rimming, then fingers and works up to his fist. LOTS of lube helps make it all enjoyable as well. He has made me cum just through his fist actions... and made me piss through his fist actions. VERY intense and hot.
Maybe 20% of the time, I get him on his back in the sling and I take control of the situation. I can 'feel' when he is in the mood to have his hole opened up, and like he does to me, I push his limits a little and keep him cumming. I enjoy those sessions because I usually fuck him for a while after the rimming and fingering. Helps lube up his hole and stretch it for my hands. He always jokes that because I am a piano player with BIG mitts, he needs to be in the right frame of mind for a good fisting episode. I totally agree.
Others have said it... the turn on is the bottom guy giving you total access... and all gates pass... to his hole. And you BOTH know that it is a relinquishing of control to the top that makes it especially exciting. Both of us love to take our time and really make the play session a good long one with lots of poppers to keep things fun. And the fact that we take turns being the guy in 'charge' of the hole means we both get to experience the action/control of the other guy now and then... and it is a real turn on to be giving OR taking.
If that makes any sense