How does a main drain work in a pool?
The main drain is positioned at the lowest point of the pool. The main drain has a perforated pipe in its base which is set in gravel in a pit at the deepest part of a pool. a pipe connects the side of the main drain to the skimmer box.
That has got to be one of the worst answers I have ever seen in this forum. Not only would there be no suction but the pool would leak like a sieve. Try taking a straw and drilling a bunch of holes in it and then try drinking through it. Main drains are connected to the pump either with a dedicated solid pipe or into the front port of a skimmer. It is controlled by a 3 port valve if dedicated or a diverter valve in the bottom of the skimmer if that type is used.
First answer, in bold type got "main drain" confused with a "sump line" as is sometimes used in areas where groundwater seeps into swimming pool hole under the liner. In such cases, a sump line is sometimes installed as described in the bold answer and connected to the pool filter pump so if water collects under liner causing liner to float away from pool bottom, sump line can be used to move water under liner to either into the pool liner, or out the waste line and away from the pool. But in such case, the sump line would never connect to skimmer and would instead connect near filter pump inlet with a dedicated valve bank to control water flow from pool or sump line since you wouldn't want to drain the pool while draining water from underneath the pool liner, and you wouldn't want pool water getting under the liner when filter system is turned off either.