For many handheld vacuum pump preservation products, once all of the air has been removed from the bottle you will hear a clicking sound come from the device. It is basically just an indication that your job is complete and the oxygen has been fully removed. If you have a bottle that has been treated this way by a Vacu Vin or similar product, you may want to repeat the process after a couple of days. Some air can escape due to the fact that none of these stoppers are 100% airtight. This will help your wine stay preserved for a little bit longer if you keep pumping out any oxygen that may have snuck back into the bottle.