Sphincter: A ring of muscle that controls the flow of liquid. The best-known sphincter is the anus. Most people don’t realize there are two urinary sphincters, a short distance apart. One is mostly involuntarily controlled, and the other is voluntarily controlled. This is why sometimes when you want to pee, it takes a moment to get started.
People who practice catheter play or deep sounding can feel the sphincters when they are penetrated. You’ll feel a sudden but sort of sweet ‘gotta pee’ feeling as the object passes through each sphincter.