if you get an overflow all you have to do is create a suction which mine is done by an aqualifter pump, when the water starts to flow in the chamber inside the tank the aqualifter sucks it up a tube then ones it reaches the peak gravity feeds it and it starts to flow over the wall and into the chamber in the back of the overflow, it goes from there down the tube that is in that chamber to the sump underneith.

it will then flow through a series of baffles which keep the water at certain heights because of skimmer requirements mainly and then it either passes through a fuge area or other area, then it gets over a piece of glass to the return pump area. the return pump area is where the water level will always vary depending on how much water your tank evaporates because the glass keeps the rest at the same height and this is the last stage that pumps it back to the tank.

the return pump then pumps it up a tube and back into the tank. usually there is a flow preventor put in line in case a power outage happens the water doesn't back feed down the line.

the overflows have ratings they can handle. say you have a 600 gallon per hour (GPH) rated overflow, you calculate what your return pump pushes, say you have a 1000gph overflow but it can only push 400gph at 5 feet which is the height from where the pump sits to the top of the tank, then since the pump pushes less than the max on the overflow then everything should flow just perfect. you should never push the limits close to the overflow ratings. if your pump is to big for the overflow it will push the water back into the main tank so fast that the tank will flood over the walls.

hopefully this helps.