It's more that "stop the flow of urine" can interfere with your brain's natural connection to the pelvic floor when urinating (something that is very much automatic and reflexive rather than intentional). Definitely a lot of people often squeeze hard and end up contracting more than just the pelvic floor, like the glutes and adductors.

You can think about drawing up a pea with your urethra, or drawing your testicles up which helps direct the squeeze and lift of a Kegel. It's also important you pair Kegels with breathing - relax as you inhale and then engage your pelvic floor when you exhale. Since the pelvic floor contraction occurs automatically with exhalation, it becomes much easier to coordinate and build the proper connections.

I just wanted to make it clear for everyone who has never tried Kegels before because often people will use the cue "stop the flow of urine" which is not great for actual function of the pelvic floor. Since this thread is about educating people on the pelvic floor and Kegels, I wanted to provide some additional info/context so someone doesn't just take that one comment and runs with it without looking too much further into it.

This is not recommended. Urination is a reflex-based act, and practising Kegels when you are on the toilet can disrupt the normal reflexes. This could cause you to develop symptoms related to micturition (urination) such as incomplete emptying, straining and difficulty initiating urination, etc.

Edit: Of course doing this once or twice will not have a lasting impact, but lots of people hear the cue "stop the flow of urine" which isn't a great way to practice (because of the above reasons) and I just wanted to make it clear for people who have never tried it before that practising on the toilet isn't really recommended. It's best to learn how to feel your pelvic floor properly from the get-go so that the habit and connection becomes easier.

I am a pelvic floor physio. Please do not just start Kegels without any guidance or an assessment of your pelvic floor. The pelvic floor is a bunch of muscles, so like any other muscle it can get tight and doing more Kegels could actually be detrimental and make your symptoms worse. On top of that, often people do them wrong and just end up squeezing everything, or they might even be bearing down when they mean to squeeze and lift (due to discoordination in the pelvic floor).

There isn't a sure-fire way of knowing your pelvic floor is tight without doing an internal assessment, and I would recommend watching a video on the pressure system and the inner core unit before attempting Kegels so you have a good understanding of how the pelvic floor works (and why you'd even want to do Kegels in the first place).