Yep, all flood systems use centrifugal aquarium-type pumps. When the pump stops running, water drains backward through the pump by gravity to return to the res.
Not depicted in my diagram in post #1 is the overflow tube. The post was about the drain system, not the operation of flood systems, which we kinda presume the reader will know if they're looking to make draining their existing flood sys easier.
SimplyHydro keeps a very nice resource online describing different watering systems.
This animated diagram should turn the lights on for you.