When is a man's orgasm not an orgasm? When it's a ruined orgasm. Yep, it's another mystery of boyparts.
Here's how it works. You give your man a handjob. You get him very close to coming. Then you let go. If all goes well, his erect penis will twitch and ejaculate. If he's not quite ready, stroke him some more and let go. Repeat as necessary. (If your man is enough of a submissive, you might get him to tell you when he's close to coming.)
You get to enjoy what appears to be a (potentially spectacular) male orgasm. He, on the other hand, will feel frustrated that you didn't stroke him to completion. (It's similar to the anal/prostate orgasm without penile stimulation that I've written about before.)
Unless you're a total femdom, I recommend you discuss this with your man and get his OK before doing it. I don't recommend springing it on him unannounced; that would be cruel. (Of course, if you are a total femdom, that's probably a selling point.)
I talked about this with The Boyfriend, and he agreed to try it. So we did. He said it was, um, interesting, but he's in no hurry to do it again.
I learned about ruined orgasms from Ruined Orgasms at the blog Caroline Cucks Michael (includes a 9-minute video of multiple ruined orgasms). Here's another video compilation.
Other posts in this series
Ruined Orgasms II
Ruined Orgasms III