The chamber fills with water pumped from the sump via return pump. The chamber continues to fill until it overflows the pipe with an upside down "J" shape. The little hole drilled into the "J" tube is to allow a piece of tubing to be inserted into the water flowing through the "J" pipe to break any siphon that would occur creating a flushing, like a toilet. The return line will be plumbed either through the other bulkhead and up to the notch in the weir. Usually there would be a nozzle allowing you to direct the flow. The nozzle can be bought online and the most popular is called LocLine.
The important thing to remember is, if you use the other bulkhead to plumb the return, the pipe must be glued or screwed with teflon tape to avoid a leak. The over flow ("J") tube must also be watertight. Now consider this: If the return pump pumps more than the "J" tube will allow to flow, you will overflow the tank. If you feel the return pump is to big, you can easily use the other bulkhead as an additional overflow(make another "J"tube) and place the return nozzle into the tank by running it up the back into the tank like a traditional return from a canister would setup.
