The arch acts like a clamp, creating a watertight seal at either end of the glass tube. This clamping action is controlled by the ball strung on the wire that runs under the tube. When the ball is in the upper position it sits in a circular cut in the arch and holds the wire tight, clamping the arch around the tube. When the ball moves down its distance to the front of the arch is decreased just enough to release the tension on the wire, allowing the tube to be removed for cleaning.