seriously, instead of talking about cups in the bathtub, here's a much easier way to grasp the concept in a way we're all familiar with.
Gravel vacs. When you gravel vac your tank, you've got a siphon going. tank up on one level, bucket or such down lower. water is flowing from your tank to the bucket, no air bubbles in there.
Imagine both ends of your gravel vac were kept under water. Now, if you were to just lift the bucket up so that it is higher up than your tank, the siphon will reverse itself and water will flow from bucket to tank.
Logically, there is a point where if you put your bucket on the same level as your tank, the siphon pauses and water doesn't move either way.
Finally, imagine your gravel vac tube was much bigger- big enough for fish to swim through. That's a waterbridge.
Gravel vacs. When you gravel vac your tank, you've got a siphon going. tank up on one level, bucket or such down lower. water is flowing from your tank to the bucket, no air bubbles in there.
Imagine both ends of your gravel vac were kept under water. Now, if you were to just lift the bucket up so that it is higher up than your tank, the siphon will reverse itself and water will flow from bucket to tank.
Logically, there is a point where if you put your bucket on the same level as your tank, the siphon pauses and water doesn't move either way.
Finally, imagine your gravel vac tube was much bigger- big enough for fish to swim through. That's a waterbridge.