Water is allowed to flow from the reservoir to the growing tray on regular intervals and then drained back into the reservoir.
The draining action also helps easy flow of oxygen to the root zone.
I think I am paranoid about overwatering. I am probably guilty of underwatering at this point. They still look pretty healthy, but I will probably shift my watering cycle from every 3-4 days to every 2 days or so. It has been relatively hot lately, and very little rain.