An external overflow only siphons water out as it is introduced from the sump. The sump controls your systems water level. As water from the sump is pumped into the tank, it spills over the pickup edge of the overflow and gets whisked away to the sump. If the power fails, the sump stops pumping water in the tank, so the tank's water level drops below the pickup edge, and the water stops flowing.
Trust me, what we are saying sounds crazy but it works, and it's near impossible to describe to someone who hasn't seen it before until they see it in action. If you really want to understand it, go to your LFS and talk to them, they'll be able to show you. You don't have to buy anything from them, they'll talk to you if they think you will.
The only other thing you need to do is drill siphon breaks into your returns. WHen you install a sump, you inevitably plumb in pipes of some kind to return water from the sump to your tank, and those pipes dip into the main tank's water. You drill a small hole just above the waterline in the pipe. When the power is on, water just dribbles out of the hole into the tank. When the power fails, air gets pulled in and breaks the siphon.
However, when this happens, SOME water always siphons back in, depending on how big your siphon break holes are. A little more water comes from what's in the overflow until that stops, etc. Thomas is saying that your sump needs to have enough extra capacity to contain this water without overflowing when the water siphons back.
Take my advice... don't mess with check valves or fancy valves that stop a siphon. It's more parts to fail, and this very simple, non electronificated way of doing it works every time... and you're hearing this from a guy who once automated his desk in the 5th grade.
