One day, I tried this while masturbating: At the moment of climax, I pressed my thumb over my urethral opening to stop the flow of the semen. This resulted in the orgasm kinda stopping part of the way through as the pleasure lingered, and when I finally allowed it to flow out again, it shot out harder as if the pressure that built up finally burst through. The resulting sensation was a much more intense orgasm than if I had simply let it flow out. I then did it a couple more times before finding out that this was a huge mistake.

Apparently, doing this causes something called "retrograde ejaculation". If your ejaculate can't escape through the front end of the urethra, it can actually flow backwards into the bladder and clog the back end of the urethra, irritating the part of your urethra where only urine is supposed to flow through. This can cause painful urination and urinary incontinence or hypercontinence, meaning you either can't stop yourself from peeing or you can't pee. There is also a potential from prostate issues, although I don't fully understand the mechanism for why or how. Luckily I haven't seen these effects myself, but I might've if I had done this more times.

What I know is: When you cum, let it all out. Don't block the flow, no matter how good it might feel. You might regret it later, and are gambling your urological health every time you do it.