Pool skimmer drain suction
Here's a picture of a pool pump that I've worked on:



See the gray valve right before the pump/basket on the left?  That's a three-way valve.  It can be turned a couple of different directions in order to adjust the flow of the water.  If you turn it one way, it will pull entirely from the drain, and if you turn it the other way, it will pull entirely from the skimmers.  In that picture, it's positioned halfway in between -- 50% drain, 50% skimmers.

If you want to pull from the drain only, you're going to have to move the three-way valve that's positioned in front of your pump.  If you have a spa, it will have two three-way valves.  You'll have to figure out how it works by messing with them.  If the pump starts making bad noises, it's either pulling in air or you've turned the valve in such a way that it blocks the pump's inlet, which is something that you don't want to do.  If it happens, turn the valve 90 degrees in either direction to open up the pipe going to the pump.