Re: How do main drains work?
It should be very straight forward: The two drains each go into a pipe. They meet at a Tee or a Y into a single pipe. This joins with the pipes from the skimmer(s) and the low drain (if there is one) and where they meet, near the pump, they should each have a valve to allow you to turn them on, off, or valve them down.
If the valves are ball valves, then the when the handle is in-line with the pipe it's open, when it's perpendicular, it's closed.
The reason there are two drains is safety. If someone dives down and clothing or hair get in the drain, there won't be an unbreakable suction--it will divert to the other drain because they are connected. Simple and effective.
If you don't have a valve for the bottom drains, then it is most likely the pipe is clogged or collapsed. The first requires a plumber's snake, the latter, repair.
16'x40' rectangle 19K gal AG vinyl pool; ; Hayward T210 sand filter; Hayward 2 Speed Superpump 1hp pump; hrs; PS 233; PF:6.3