These specially designed, hand actuated cups operate on a pump system, and create a vacuum under the rubber pad.
The pump's plunger has a red-line to indicate vacuum level at all times. When it shows less than 100% vacuum, simply pump the plunger until you reach the original vacuum pressure without removing the cup from the glass.
To release, just depress the release lever on the opposite end of the handle from the plunger.
These cups come with a release valve guard to prevent accidental loss of vacuum.