Preformed loops are used with loop detectors to detect the presence of vehicles in your driveway or parking lot. When a vehicle passes over the loop, it breaks the circuit and the loop detector will send a signal to one of your relays in either your garage door opener or gate opener. This relay can do one of several things, allowing free exit through the opening, or sensing a vehicle and the loop detector will not allow the gate or garage door to close on the vehicle. The loop detector can also be wired to keep the gate or garage door open until the vehicle is in a safe zone. Used with a timer the loop detector will not allow the timer to begin until the vehicle has moved out of harms way.