This is a fun one. To the uninitiated, "post orgasm torture" may summon images of a man being punched in the balls after he's come, but it isn't that at all. In actuality, the things done during post orgasm torture would be very pleasurable pre orgasm. After orgasm a man's cock become very sensitive. More specifically, what becomes sensitive is the glans, aka the cock head. As a woman you've probably experienced this with your clit. Touching it directly after orgasm is just too much. Forcing this sensation on someone is post orgasm torture.
One important thing to know about post orgasm torture is that it causes involuntary motion. Think of being tickled, you can't help but laugh and squirm. In post orgasm torture these reactions manifest themselves in sudden jerks, flailing, and sometimes even convulsions. While sexy as hell for a sadist like myself to behold, they are quite effective at stopping you from playing, and can even be dangerous depending on your distance from said limbs. I recommend you always securely bind your sub beforehand. Not only does make the act more "forced," but it lets you safely go on as long as you'd like without being interrupted. Also, it's sexy to see someone straining against their bondage.
Prolonging the erection is important, as blood will quickly drain from the cock after orgasm, especially during post orgasm torture. Go to work right after orgasm, don't let the cock sit for any amount of time. The harder the cock, the more full the head is, the more sensation it will feel. Cock rings or cock bondage can slow a falling erection (set that up before orgasm), but all you really need is to grab the cock shaft near the base, pull the skin tight, and squeeze. This will can slow and sometimes even stop the blood from leaving the erection. I suggest using your non-dominant hand for this, so your dominant hand is free to do the dirty work.
The ironic part of post orgasm torture is that the more pleasurable an action would normally be before orgasm, the more painful it will be afterwards. Lubrication increases sensation, so a good tip for post orgasm torture is to use lots of lube. Cum will act as a lubricant, but I suggest having a bottle of lube near by and making sure he is lubed up good before orgasm. Secondly, the more constant contact you are making with the glans, the better the reaction you will get. Thirdly, don't hold back! This is entirely harmless fun. The reactions you get may be over the top and uncontrollable on behalf of your pet, but again, compare this to tickling. This is actually one of the safer "torture" acts found in kink.
The most common method for this is to just stroke up and down quickly over the head of the cock. Squeezing as you go increases contact and sensation. An easier method, and one that also tends to give better results, is palming. It's pretty simple, close your palm over the flat of the penis head. Run your palm in circles back and forth over the glans, rubbing the tip, the flat, and the rim all at once. If your hand is small enough you can even let your fingertips brush the sensitive spots on the under side of the cock head, but focus your pressure primarily on the flat. You can also do this method in reverse, focusing on the underside of the cock, but for some reason it seems to get less of a reaction. Any hand job technique will work to some degree, and everyone is different. See what makes your toy jump the most and repeat that, or try to find new ways to solicit such reactions. Just have fun with it. You could even make a game out of finding what gets the biggest reaction from your sub.
The best thing about post orgasm torture, as I've mentioned before, is that what you are doing would normally be extremely pleasurable. This is part of the fetish for those who are into it - forcing someone to endure something that they would normally find enjoyable, and for those on the other side, being forced to endure such things. If you want to play off of this element of the fetish you can tell your sub thing such as, "But this is what you like!" Or, "If you don't like this then why did it make you come?" The "you like this" angle is fun. Depending on the sub, you could even use post orgasm torture as a punishment, perhaps for coming too soon, or as part of a game. "I'm going to play with you for X minutes, if you don't come you lose your orgasm for tonight, but if you do come I won't stop until the time is up."