I've had a few guys have what you described while I was fucking them, two of them were regular fuckbuds and, in addition to them shaking and having their eyes roll back, their holes would absolutely flood with lube, pure white and smelled like cum but in a huge quantity. I do concentrate on the prostate some of the time, but switch it up and also do deep strokes and long strokes pulling fully out and back in. One of them it didn't seem to matter what I was doing, and he had them over and over every couple of minutes the entire time we were fucking. The bed or floor under us would be completely soaked with the cum like fluid from his ass long before I came. The other guy typically had the as soon as I would start deep dicking him after a few minutes of prostate play, the 1st time it happened he was sure I had bred him, and when he found out I hadn't he made a dash for the washroom because he thought he had had an accident, but what was coming out always looked smelled and tasted like cum.
Unfortunately I've never experienced one personally, I've definitely had prostate orgasms where I cum hands free while I'm getting fucked (second time I was ever fucked and can happen unexpectedly anytime I'm taking a good hard pounding), and I produce more than enough of the cum like ass lube to get fucked without requiring more than a spit and shove to get a fuck started, but I've never actually felt the assgasasm that I've seen a few bottoms have.