For example, suppose your horn has suddenly given up the ghost, you have no test light or voltmeter handy and you'd like to test the circuit before shelling out for a new hooter. The question is whether the horn failed on its own, has a bad ground or is not receiving current. Because a bad ground is always a prime suspect, you can use your jumper to connect the horn's ground terminal directly to a known good ground. If the horn blows, great; if it doesn't, take a second jumper and connect the horn's 12-volt terminal directly to a known good power source (the battery's positive terminal is the preferred choice). Still no horn? Well, that bites, but at least you know the circuit is working and you won't be wasting your dough on a horn you don't need.