People of all sexes sometimes find this act, sometimes referred to as ‘watersports,’ surprisingly erotic. Some people play with it. One version is to hold urine as long as possible, resulting an a spectacular flow when they finally do urinate.
Another form of play, generally for men, involves blocking the flow of urine. One way is to wrap the end of the penis in cloth to protect the skin, and then bind it tightly with string or rubber bands. A simpler version is to simply squeeze the meatus (peehole) closed with thumb and forefinger. Yet another version is to bind the penis just ahead of, or behind the scrotum. In the latter case, the spermatic cords are also bound, which can be a bit dangerous.
What happens is that at first the urethra fills with some urine. You can see the penis, whether hard or soft at the time, swell and curve upward. After a moment, if the urine isn’t released, the body starts involuntary contractions to force it out. Many men have never experienced these contractions. They are much stronger than orgasmic contractions. These are remarkably powerful and can be quite painful. The pain usually occurs where the the penis is bound as the urethra is stretched by the pressure. Unless the binding is very tight, some urine will be forced out of the penis.
If the binding is too tight, you may be causing a problem, as the contractions may be too powerful, causing extreme pain or damage when the urine cannot escape.
Human evolution probably developed this strong spasming or contracting reaction to help rid the body of bladder stones.
This practice may be dangerous, as unnatural forces are built up, potentially causing damage to the urethra, ureters, or even the kidneys.
A safer version is to squeeze the end of your penis until it fills with urine, then let go so a heavy-duty splat of urine flows out. Then squeeze it again, and release another high-pressure, high-volume spurt of urine. Continue, and perhaps vary the frequency, so you can have a sort of pulsating urination.
Uncircumcised men enjoy urinating into their foreskin, temporarily held closed with thumb and forefinger.
Your author is inexperienced in female urine retention, but quite possibly women would also enjoy playing with the idea of blocking the flow of urine.
You might also be interested in peegasm, the skill of ejaculating and urinating at the same time.