Pinching Pee

So, I was walking the dog the other day, and its peeing practices interested me. I was wondering, in my brainsicle, if it is easier for dogs to pinch off pee than it is for humans. When I'm peeing, I can not pinch it off without, at least, a little bit of pain. I know I certainly couldn't do it, if I knew I had to walk a ways more before i could finish. These dogs just seem to be able to pinch it off with absolutely no trouble. They just keep walking like there's nothing to it, even deciding from time to time to go some more, pinch it off again, and keep walking. They seem to be able to keep this up for miles on end. Even on long walks they tend to hold at least a little back for the last quarter mile. Simply amazing.