Similar to a woman's orgasm, I would say it's 50/50 mental stimulation and physical stimulation. In addition to the prostate, I think that anal orgasms also have something to do with the constant stimulation of the sphincters and blood vessels in the ass. It's a shame that there's not more writing about this on the internet.
For me they can vary in intensity, but my cock usually isn't hard and I don't ejaculate when I have one. My whole body, but especially my lower body spasms uncontrollably, I get really warm, and my sphincters contract and expand rapidly. Afterwords I'm usually overtaken by this feeling of both contentness and vulnerability. The best thing about them is that I can have multiples!