The prime a jet threads into a 1/4 NPT hole on the top of the jet pump bowl. It is designed to release the air that gets trapped in the pump during initial boat launch.
If you have to to rev your engine to get the boat moving after loading it in the water, you have air trapped in the pump. The PJ bleeds off the air so you are ready to go. It leaks air out and closes as the water pressure in the pump increases.