Imagine...  You unload your boat off the trailer and fire the engine.  When you push the throttle to accelerate, the engine revs but the boat sits still.  This is the result of air trapped in the pump.  This little air bleed threads into the top of the pump bowl and bleeds out the trapped air.  It mechanically closes as the rpm increases and requires no controls or maintenance.  The Prime a jet also saves the wear ring from continuous dry start damage.  It is simple to install and comes with instructions.