See also Performing a Cylinder Leak-Down Test in the REF section of the Sportsterpedia.
This is basically a flow meter showing the differential pressure between the two gauges.
Simply put, the left gauge measures incoming pressure from the air tank and the right gauge measures the pressure lost thru leakage in the cylinder.
Some have dual psi gauges for which you can induce a dedicated 100 psi of air into the system which makes deducing percentage easy.
Others will have a percentage based gauge on the right (with a lighter calibration) using a pre-determined amount of air via the regulator.

• When using the differential pressure gauge set, ¹⁾
  • The first gauge shows the regulated pressure.
  • Then, the air passes thru an orifice (a smaller hole inline between the two gauges to restrict the flow between them) to the second gauge.
  • The air then leaves the second gauge to the cylinder via the spark plug adapter.
• This pressure (end to end) would ideally be the same if there is no cylinder, valve or head gasket leakage.

This test will also tell you what to look for at the top end of a tear down: worn exhaust valves, worn intake valves, leaky head gaskets, or worn rings. ²⁾

1. Turn on the compressor, connect the gauge to the air pressure, and adjust the gauge.
2. Thread the adapter into the spark plug hole and attach to the gauge.
3. The gauge now shows the amount of leakage.
4. Interpreting the readings
5. Hunt down the source of leaks

Commercially available leak down tester (from Harbor Freight) 3)

Homemade Leak-Down Tester 4)

Homemade:

An old spark plug with the ceramic removed can be used to make an adapter for air hose fittings.
Ceramic is too hard to drill and it's easier to just break it off flush with the metal and cut the compression lip off the plug.
With the plug hollowed out, an air hose nipple can then be welded or epoxied in the hole to connect a regulated air source ⁵⁾.

Homemade air nipple adapter. 6)