A “Leakdown Test”, also know as a “Differential Compression Test”.
Practically never seen in automotive repair shops, this test is the only compression test you’ll see in an aircraft repair shop.
Does our narrator fully understand the leakdown test? Correctly performed the engine should be at TDC. On true TDC it won’t spin. The test can be conducted at BDC as in the clip, but leakage pressure reading will not be as accurate. (Some aircraft engine manufacturers publish a minimum pressure for a cylinder to be deemed “serviceable”.) But more importantly interpretation. In the “ideal” cylinder there is no leakage - and thus BOTH gauges will read the same. However, there is no “ideal” cylinder, so the second (“leakage”) gauge will always show a lower pressure - the difference in pressure between the gauges being proportional to the amount of leakage. The supply gauge (nearest the regulator) feeds through a calibrated orifice allowing it to show a set feed pressure (in the clip 90 psi was used, in aviation 80 psi is standard). As leakage increases the cylinder can not maintain pressure and the pressure on the leakage gauge drops - more leakage, lower pressure on the gauge. In the extreme, say a broken valve, the second gauge may never read any pressure (0 psi). A good tight cylinder may likely read within 20 psi of the supply gauge pressure.
The narrator opens citing 2 sources for an oil leak. But, as noted later in the clip, there is a third source of oil leakage into the cylinder - the gaskets. On GM engines and similar engines that use the intake manifold to seal the valve valley, as well as engines which combine the valve valley cover with the intake manifold gasket, an intake gasket leak can contribute oil. This leak will also contribute a vacuum leak resulting in an excessively lean mixture. All those gray deposits on the spark plugs? But note there is also a pressure port bringing oil to the head to lubricate the valves. A head gasket leak may spray oil into the nearest cylinder.
Running the engine down to 2 quarts oil! And knowing it burns oil! Someone needs to be whipped with a wet noodle and deprived of ALL driving privileges!