I'll give you my experience on what it feels like for me to bottom and cum from just anal sex alone, without stroking my cock.
To preface, what works for me is not universal. But I enjoy a more average sized dick. Especially a nice, fat head on the end - perfect for hitting my spot. If he has an upward curve, fuck yes. Whether missionary or riding him on top, I find face-to-face allows the best angle to hit my prostate right, plus it's just hotter and more intimate and he can thrust better.
The entry is intense sometimes, as one top mentioned above. It either involves a brief pain from the spasm when my hole clenches down, or it slides in really really easily and completely and utterly painless. Usually it's somewhere in between for me, but more on the painless side. I found if I lube my hole good before he goes in, rather than just putting lube on his cock, works best. If my sphincter is really tight or there's any pain, usually pulling out almost entirely then going back in relaxes it.
There's a point after a few seconds when the internal sphincter, which is under involuntary control, just relaxes completely. This is when the whole game changes.
Those first strokes after my hole relaxes, taking his penis all the way to the balls, is a feeling that lacks adequate words to describe. It feels right on a primal level, like this is what I'm supposed to be doing. Taking a dude's dick up my fuckin ass. It literally takes my breath away for a moment. It feels perfect. Extremely satisfying. I love to grab onto his thighs or ass and pull him in and just hold him there. It's like the ultimate male-bonding experience, the closest I can be to another man. By this time he's usually throbbing a bit from his own excitement, which is just awesome to feel inside me. The entire moment - the intimacy, the vulnerability of it, the eye contact and smiles, the letting go mentally, holding onto another man who I'm really into, the male energy between us - is just as good as the penetration itself.
His dick sliding back and forth feels good against my hole, and the sensation of him filling my rectum is incredible. But nothing compares to how he feels against my prostate.
Holy shit.
From that first relaxed stroke, it's like I'm on the edge of cumming the entire time. I can feel the ridges of his fat glans slide across my prostate. It sends me into a sort of trance. My dick stays rock hard from it. The mechanics of the fucking really depend on the mood. Sometimes I love the sensation of going slow, him being intuitive and figuring out where to hit and slowly sliding back and forth across my prostate, which by this point is engorged and hard, making it easy for him to feel and hit just right. Other times, I like to ride him and work his cock, being in control, grinding his cockhead hard and slow against it. And other times, I just want him to hit that spot as hard and fast as he can and just fucking crush my prostate. No matter what, I am rock hard this entire time. I don't jerk off, I feel like it "transfers" the sensation from my prostate into my dick and it's not the same.
This is when being really into it is key. The hornier I am, the better. I can turn into a fucking animal. This obviously depends on your personal preference and what turns you on. The sensation overwhelms me. As I said earlier the whole time feels like I'm on the verge of an orgasm, like perpetual edging. Cock rock hard, leaking pre cum like crazy. And then that feeling finally happens - I know if he keeps going, I'm gonna cum. At this point I've already lost control. I am probably making noises I didn't know I could. It's like I am incapable of what's happening to me.
And then it happens. A pure anal hands-free prostate orgasm. I hold on for dear life.
First I feel it in my prostate. It's like a white-hot, internal feeling. Imagine all the best orgasms you've ever had from jerking off. Now package that into a super dense ball, radiating from inside you. I feel it deep inside my rectum, which clenches down hard on his cock. My anal sphincter becomes a vice grip. And I just fucking explode. I cum HARD. I shoot huge ropes of cum. The pleasure radiates out from my prostate and feels so fucking incredible in my ass, traveling up my hard dick, pulsating as I shoot. Sometimes my legs shake. I've almost blacked out during it. I can't adequately describe how good this feels. It does not even compare to a penile-only orgasm, not even close. It is so much richer, deeper, and more satisfying. And for me, and why I love anal sex so much, is that every time this happens to me it feels just as good as the first time.
Tops don't seem to last much longer after this. I love when tops also lose control and moan really loud as they're about to cum inside me. The hottest sex I have is when the top grabs onto me while he cums. I can't feel the cum hitting my walls inside because we don't have the nerve endings for that but I feel the warmth of it and feel him pulsing. My favorite is when he pushes in all the way when he cums, so I feel his tight balls right against my hole and can feel them pulse against me. I love when tops leave it in deep and hold it until their last cock quiver fades.
It honestly might be the best feeling there is.