As a top, I LOVE fisting and anal stretching. And "pegging", and prostate play, and... you get the idea. I could gush for hours about it.
My primary fetish is depravity, and anal play hits all my buttons in that way. It's also really cool to see what the human body is capable of, stretching (heh) it to its limits.
Contrary to its portrayal in porn, It's an incredibly intimate act that requires a lot of trust, patience and skill. I am always aglow when I receive demonstrations of devotion and trust from my submissives and bottoms. For those of you that have never fisted someone, you should know that you can feel the bottom's heartbeat when you're in there!
I practice tantra in my personal life, and one of the things you focus on is transference of energy. I can pick up on other people's orgasmic sensations, and have had experiences similar to the one you mentioned your dominant experiencing in your post. Simply put, if we're very present and connected when we're getting it on, I might just pick up orgasmic "aftershocks" myself, without being touched. I also get off psychologically on getting people off, especially huge, powerful, full body prostate orgasms, the kind that leave you buzzing for hours.
Aside from all the sexual gratification, I always have a swelling of pride and appreciation for my bottom/submissive when I finally get that last knuckle in and slide the rest of the way in...
Uhh, that's all I have to say about that for now, I'll be in my bunk.