The unit is activated when the internal microphone picks up constant barking sounds from the dog. Once activated the unit emits a 2 second ultra sonic sound that can only be heard by the dog. Startled by the high pitched sound, they should stop barking, associating their bark with the unpleasant noise. Use the manual correction button for unwanted behaviors, such as lying on the furniture or getting into the trash. If the button is held down, it will repeat the ultrasonic sound in 2 second intervals. The unit should be 10 feet or less from the barking dogs and 20 feet or less in manual correction mode. Operates on one 9-volt battery (not included) which will last 6 months or 2,000 activations. Low battery light flashes once every 6 seconds when battery needs replaced. Non-returnable 3 days after receipt.