The vertical tube is plugged into the hole that goes out the transom. When anchored or drifting, water comes in through the tube and fills the well. The float switch cycles the pump on and off to maintain the water level. When I want to drain the well, I pull out the tube when the boat is on plane or on the trailer.
The hole to the port side of the tube is connected to a clamshell through hull pickup. There's a pvc ball valve in the line that I use to control how much water flows into the well while the boat is on plane. As water comes into the well on plane, it overflows through the vertical tube and out the transom. If water is flowing in faster than it can flow out through the vertical tube, the float switch will activate the pump.
Dave