In reality you cannot use the pool pump to drain a pool through the main drain. It’s too easy for the plumbing and pump to lose prime and, once that happens, you can damage the pool pump by running it dry. Also, given the pump/plumbing limitations of the main drain line, the flow rate of water would be too slow. One or two submersible pumps or, even better, a gas driven trash pump could easily drain a pool in less than a few hours while using the main drain could take all day.