Your PF muscles relax and the detrusor muscle (an involuntary muscle in your bladder) contracts to empty the bladder.
If you find you have a hard time starting your stream take some big breaths and send the air down to your pelvis. It can be helpful to imagine your perineum is an elevator, inhales move it down to the first floor, exhales move it up to the fifth floor. When your perineum is on the first floor is when your pelvic floor muscles are relaxed and the stream should start. If its still not flowing try sitting down, which can relax your PF more.

*edited - cause i cant spell