It’s common for us to talk about stopping the flow of urine to identify the bulbocavernosus (BC, but also called the bulbospongiosus) muscle. This muscle is involved in the ejection of urine from the urethra, but it is not the one used to stop urine flow. That one is a sphincter muscle (a ring muscle) around the neck of the bladder where the urethra exits to the outside of the body. It is a passive muscle. You contract it to OPEN it and let urine flow. Relaxing it automatically closes it. But talking about stopping the urine flow is an easy way to help someone identify the BC.
Another way is to get an erection and make your penis bounce. Contract the muscle that makes it jump. That’s the BC. No urine involved, no confusion. If you’re making your erect penis bounce, you’re contracting the BC. The BC is the muscle to exercise for firmer erections, a more forceful ejaculation, and one that can be used when you want to prolong your orgasm by not ejaculating. The exercise that does this is the Kegel (as talked about in the first post of this thread).
Contracting the anus when contracting the BC is common. The nerves that transmit the impulses to contact the muscles are all interconnected and contracting one thing usually contracts another. Over time, with practice, you can separate the different muscles and eventually learn to contract only the BC. As far as the other muscle you’re contracting, the one that is affecting your urine flow, I really can’t say what it is. The BC can be felt from the outside. It’s between the scrotum and the anus. If you’re contracting something higher up and inside the body, then it isn’t the BC. You probably aren’t hurting anything, but if your urine stream is not normal then you probably should stop exercising whatever it is that you’re contracting.
BTW, urine is expressed from the bladder by contractions of the bladder itself. You can help it along by bearing down (like for a bowel movement or when holding your breath and contracting your abs), but it usually isn’t necessary to do that.