Read some of the live coverage of the big races (say at
www.cyclingnews.com). Notice that once the break 'sticks', the gap goes up by 3 minutes in about 5 km. There's your piss break.
They also go while moving, although my understanding is it's 'over the top' not down the leg. I've tried a couple of times: the first time I had stage fright (even though I was by myself) and couldn't go. The second time I only managed to wet my shoe a little.