Glenn - here's a page by Melev that has an animation of how a sump works, and what happens when the return pump shuts off (or is on). Note the tiny circle at the top of the return line on the left side - that's the siphon break hole mentioned by Clownfish518 - when the return pump is powered off that line will only drain the tank till it gets to that hole, which allows air in the line and it stops draining the tank at that level.
On the right side is the siphon box (such as the LifeReef overflow), These have an inner & out box, with a u-tube that siphons water from the tank through the inner box, through the u-tube to the outer box, & then gravity fed to the sump below. When the return pump shuts off, the water in the back of the RSM stops entering the inner box when it gets below the edge of the inner box. The u-tube doesn't lose it's water in the LifeReef, and when the pump comes back on, the water level rises again in the inner box and restarts the gravity feed to the sump again.
Hopefully the animation will help make sense of all this:
Melevsreef.com | Acrylic Sumps & Refugiums
It's actually a pretty simple system, with only one pump needed of the proper size (and a good, fool-proof siphon overflow like the LifeReef).
I love it since I can run my skimmer, reactor & ATO in the sump, and I have a constant water level in the DT and in the back - and zero microbubbles.
