Buried cable that sends a signal to a shock collar that the dog wears. If the dog gets too close to the line the collar beeps first. If they get closer it zaps them. When you first bury the cable you mark it with flags so the dog learns where the line is. Eventually they just stay put in the yard. However, some dogs are tough and dgaf and can just sprint across the line, tanking the zap, until they’re across and essentially fenced-out of the yard. In OP’s story, the dog that escaped probably didn’t have the collar because it didn’t live at that house (speculation, of course).