Wood's Powr-Grip Vacuum Cups are specially designed, hand actuated cups that operate on a pump system, and create a vacuum under their gripping pad section. The pump plunger has a red band to indicate suction level at all times. When it shows less than 100% vacuum left, simply pump the plunger until you reach the original vacuum pressure without removing the cup from the glass To release, just depress a trigger on the opposite end of the handle from the plunger.  8" (203 mm) diameter Durable metal handle Vacuum-molded carrying case included to protect the cup when not in use Fast attachment on smooth, non-porous flat surfaces Red-line indicator warns user of any vacuum loss Check valve allows re-pumping without loss of remaining vacuum Release valve lever permits quick, complete release