Indoor Pet Fence systems emit a radio signal that is detected by the receiver collar worn by your pet—the same way the outdoor Hidden Fence systems work. As your pet approaches the signal field (initially marked with a series of flags to help your pet identify the area), the receiver collar begins to emit an audible (or vibration) warning beep. If your pet goes beyond the warning zone, the warning is quickly followed by a mild ‘correction’—an unpleasant, but safe, sensation from the receiver collar. With proper training, your pet will quickly learn to stay away from areas that he associates with the warning.