It's not a secret. It mainly depends on the water being very near the edge of the level, especially if there is no wall around it. The box seems to scoot off sideways, and although it disappears from the water, you can still walk on it.
If you want it to happen then it's good, but if you don't then the invisible box just gets in the way.
I don't think there's a hard and fast rule for getting one to appear, you just have to experiment. Whatever you did last time in the editor needs repeating, then you'll most likely get the same result.
