Never done it but i got close. I refused to touch my cock for nearly an hour while i fingered myself. It's a crazy feeling, not the same as a regular orgasm. I can't tell you how it feels but i can tell you that my vision got kinda fuzzy and i started seeing stars and squirming and moaning despite trying really hard not to. I'm usually a stealth fapper so moaning in general is something i don't do.
Anal is weird, it feels like pooping at first but give it about twenty seconds of fapping simultaneously (or just being horny) then it starts feeling like a fap and the poop sensation goes away.