It has 2 parts aligned against each other with little space in between them. One part is mounted on the overhead moving garage door and the second part is mounted on the frame or on the floor “made of steel” so a car can pass over it. If someone tried to pry open the garage door the two parts get separated and that sends a wireless RF signal to the Control Panel/keypad. It has a special long life battery that can last up to 5 years.