The pump housing will remain full of water while the pump is on, and will remain full or partially full of water when the pump is shut off. When you turn on the pump the motor will begin to rotate on high speed, even if you have a dual speed pump. The motor drives the pump impeller, located inside the pumps center portion at the opposite end away from the electrical switch portion of the motor. While the motor is rotating, the tips of the impeller are sealed hydraulically inside of the pump diffuser, this allows self-priming to occur. Self-priming can only happen provided the pump has a diffuser.
Some pumps have a separate diffuser, others have the diffuser molded into the pumps cover.
Self-priming pumps are very dependable and simple in design. They require a sufficient supply of water from the spa, and no air in the suction lines. Air could come from a loose strainer cover, a leak in any valve, a pin hole in any suction line or any crack or loose connections in the piping. Your pump should be kept free of dirt and also located where it can be protected from flooding during heavy rain fall. If your pump motor becomes flooded you may have to replace it.