The leak down test involves applying pressure to each cylinder while valve is closed. The percentage of pressure lost is monitored.

The most valuable part of a leak down test is in the case of a low compression cylinder. You can apply the pressure to that cylinder and listen for where the pressure is leaking. If it is heard through the oil filler cap the leak is past the rings. If heard at the intake manifold, the leak is an intake valve. If heard out the exhaust, the leak is an exhaust valve.

Hope this helps,