Brettski is correct. For a siphon you must have a greater amount of water going downhill by gravity than a greater amount of water coming uphill and you must have a closed system (i.e. no air). The vent is a siphon stop (if the vent was not there the pond would drain down to the level of the inlet until air was introduced). Once the pipe fills up (and it doesn't have to fill all the way, I'm guessing only 1/2 from what I've been told) the siphon kicks in draining the pond. Once the water level comes back down to just below the vent pipe, air is introduced and the siphon stops.
