If the warning is ignored and the pet crosses beyond the boundary of the fence, the pet receives an additional signal, such as a mild electric shock, or a combination of signals that causes the animal to adapt its behavior and remain within the boundary.
An underground fence is an electronic system to prevent pets from leaving a yard.
A buried wire around the containment area emits a radio signal to activate the receiver collar.
In another type, the collar uses GPS signals to determine proximity to a predetermined "virtual fence", without the need for any physical installation at all.
This type of fencing does not have a warning mechanism to humans who might inadvertently wander inside the perimeter, making them more susceptible to dog bites/attacks.
This type of containment is also not maintenance free; this system can only operate if the batteries in the animal's collar are properly working.
In August 2018 the British Government announced that all containment fences should be professionally installed to ensure the safety and security of both cats and dogs.