Good question and I'm sure its one several members here might wonder about.
My prostate spasms for sure, but its also my anus and rectal canal, everything down there and inside pulsates like crazy just like it does when I have a traditional ejaculatory orgasm with an erection. But my penis pulsates too, it more so moves and flexes, like when you can see your heartbeat make your penis move sometimes, it pulsates and jumps a little like that. In rhythm with my prostate orgasm.
My anus clamps down on the toy in me (Aneros or any hands-free prostate toy) and I'm like saying inside as my body gets a bit rigid and tight against my will "oh shiiiiiit fuuuuuck its coming!" and then when it relaxes on its own, again I'm not controlling any of this, the pulsations start and my orgasm hits and it washes through my whole body. Centered in my pelvis and it radiates out into my torso. Prostate Os last like 5-20 seconds most of the time for me. Sometimes they happen just 10 seconds apart from each other.
So my prostate orgasms are intense and hard, but like "internally" not "externally" like when I'm having sex and pull out to shoot cum all over my wife. That's my hand driving that, and its very internal but the cumming happens because of what I'm doing externally, via my hand on my cock. I can extend my orgasm that way for a long time up to 45+ seconds most cumshot orgasms. I can't extend or shorten my internally driven prostate orgasms at all. They last as long as they want to. I guess it depends on the toy, how horny and intense the stimulation is, and how far into my session I am. After about 20 minutes in my prostate Os usually last longer than 15 or so seconds. Maybe up to 30 seconds, those are the Super Os though, I only have a few of them. Extreme "moment of clarity" and bliss, tired but present and awake, just washed over and done feeling. I have to lay there with the toy in me for a minute or two before anything else starts to happen on its own or by my own will. Once I have a Super O with a toy I usually change it out for a different toy. I use no less than 5 toys a session: Aneros toys, glass g-spot wand, and 1-3 realistic dildos to ride. I get pretty far along and my need for traditional release takes over like "yep, I have to cum RIGHT NOW" and I put a final Aneros in me and lay down to stroke myself until hard and shooting all over myself. These final Os, Super Ts, are fucking mindblowing and ultimately draining. My last session ended this way as they all do and I was so worked up I timed my final O and cumshot so good I blasted a massive rope right at my mouth but it was closing as I shot at my face and a little got in my mouth and the rest splattered me from nose to below chin!!! It was so great! Very fun stuff