Dave,
I don't know if you've seen a pic of the set-up but imagine a pipe of suitable size running from the bottom of a pond up & over/through the dam but lower than the spillway, down the back side all the way to the base. A tee is installed on the top front of the dam in the (main) pipe & a smaller pipe (vent) leaves the tee parallel to the water & turning down 90 deg toward the water. Wherever this pipe ends the water level will be.
As the water rises it blocks this smaller (vent) pipe & eventually rises high enough to start running down the larger (main) pipe. Once this starts it creats a siphon & will continue until the water gets below the smaller (vent) pipe & the siphon stops.
If you plug the vent pipe the siphon will continue until the water gets below the intake of the larger (main) pipe.
Hope this explanation is coherent.
