The technology is radio frequency technology. The offender wears a tag on his ankle, which sends out a signal to a transceiver installed in his home. The transceiver picks up a signal from the tag on the ankle and sends it to a monitoring centre, either by a telephone land line or by the cellphone system.
There is a person in a monitoring centre who is able to know whether that transceiver is picking up a signal from the tag on the ankle or not. If the offender is supposed to be in the house, say, between 7:00 at night and 7:00 in the morning, then the monitoring centre will know whether he's there or not, because the tag on his ankle is sending a signal to the transceiver.
If he leaves the house during the period that he's supposed to be there, or if he tries to remove the tag, the monitoring centre will know that happens.
This technology is tried and proven.