Some more thoughts,

Vagina
Post-orgasm torturing with the vagina works well if the vagina is nice and tight. A youthful or at least well-exercised tight vagina can do a good job of this, especially if he is sensitive. But it might not be as good at being 'cruel' as the fist. The vagina method feels sexier. It hits the whole penis, not just the hypersensitive parts. If he's hard and that erection has restablized, riding him might be good for extracting a second ejaculation for him. It's probably best if you use a penis ring if you are going to ride him. My wife is petite enough, and I like her to lay her whole bodyweight on me so I can really feel her skin, and that ads to the erotic feel of this. There are probably pheromones and oxytocin there adding to how bonding this experience is. But it isn't the best, probably, for torturing the head in a 'cruel' way.

There is something intimate and special about a woman post-orgasm torturing your penis while she's loving you with her vagina, and she's hurting you the right way.

Using a Stroker
If you have a penis stroking device, like a fake vagina in a plastic cylinder, you can use it to reduce fatigue for a long session, stimulating him. It might also catch the semen pretty well. But for the post orgasm torture, quickly remove the penis from the stroker, and give it to him with your fist. It probably isn't going to torture him as well as a hand will.

Sometimes the stroker, since it stimulates parts of the penis at the same time, is good for getting that second ejaculation out of him. But again, after that second ejaculation, you are going to need to use a well lubed hand on that penis head to really post-orgasm torture him properly. You can keep at it until he is soft and then for a minute or two after to make sure you've used up all the hypersensitivity.

Yelping and Writhing
If he is sensitive and you do it right, there should probably be some yelping and writing. You could give him a post-orgasm handjob where it's just a little too much and he can't decide whether he wants it to stop, or part of him does. You could torture the sensitive bits and make him wish it was over. IMO, where you want to have him is in that undecided stage for a second or two, then that 'want to stop' stage for a second, then back down to undecided. He may get out of that, slowly move toward orgasm, then you repeat the process. You can go for a third orgasm if he's up for it, then stroke some after that to let him experience the torture. When he recovers, get feedback about which was more intense, after orgasm 1, 2 or 3.

Part of the experience is making him experience something he regrets in the moment. That's okay. It's kind of the point. But you are giving him a really sexy memory for later. And part of him may crave what you are doing while part of him says it is just way too much. He's tied up so you are making the decision for him to keep going. If you ask him if he wants to stop, he probably will, so the onus is on you to make it happen so he can experience his fantasy.

Cramps and Heart Attacks

If he is young with no heart problems, this may not be an issue. For a long session, keeping him hydrated may help. Gatoraide, poweraid, maybe even coconut water (the potassium part but not sodium) might help reduce the chance of cramps. If he's gagged, you might not be able to tell cramps from writhing. Massage him first if he has sore muscles. An electric back massager is good on backs, calves. Make sure he is strapped down in a comfortable position.

Choice of Rope
You can look up how to bind a man. Something that we played with a little was I taped both sides of some velcro straps to each other with a key ring in the middle, tied rope to it, and made under-the-bed restraints. I had similar straps for my wrist, which I strapped to the outside of my thigh. Post orgasm torture was just a little too much for me when I did this, and I could take it enough to not knock her off the bed. I just needed help keeping my hands off my crotch and keeping my thighs open. So how you tie him up depends on how sensitive he is. The really sensitive young men may need to be tied down more securely. Muscular athletes may need to be strapped down even more securely.

Testicles Spanking and Second Orgasm
When I have gotten stuck in an erect refractory period where the feelings weren't climbing, some light ball spanking accompanied by the hand job might be able to get me out of that and on track to a second ejaculation.

Lost Erection
If he loses an erection, you might be able to get one out of him with a penis pump, or with a skilled mouth. But he might not be able to keep a full erection during his refractory period or ejaculate soon after. This can be fun with a penis ring if you want to ride him for your own pleasure and you can get him hard. He's tied up, and if he's agreed to the experience, you don't have to worry about his ejaculating too soon, or if he does, just enjoy his reaction as you keep on going.

Every Experience Different
You can do pretty much the same thing and you might get a different reaction. He might not be as sensitive as the last time. He might ejaculate soon after you start post-orgasm torture, or he may lose an erection. He could get into a hard refractory period, or not.

Viagra Penis
Viagra can make a post-orgasm penis be hard and be less likely to feel any sensation. Usually, men on viagra are older, too, and may not be as sensitive post orgasm as they were when they were younger. A really sensitive man on viagra might be sensitive still, post orgasm. It depends on his reaction to the medication on this particular occasion.