Rentokill Pestfree Cat & Fox Deterrent The Rentokill Pestfree Cat & Fox Deterrent protects your garden using ultrasonic technology, which causes cats, dogs, foxes, and other animal pests to avoid protected areas, while the deterrent cannot be heard by humans. Waterproof and easy to use. How Rentokil Pestfree Works 1.Motion sensor detects movement in the protected areas. 2.The unit triggers a 10 second ultrasonic burst, which will repeat until the animal moves and leaves the protected area. 3.A red light behind the lens indicates the unit is triggered. Why It Works 1.Unlike many pest deterrents that operate continuously, this unit waits for the pest to enter the protected are. 2.internal sensors detects the crossing of a warm body in the field of view 3.powerful ultrasonic sound burst startles the pest. 4.The animal learns that it is causing the unpleasant occurrence to happen every time it goes into the protected are and therefore stays away.