What is this test, I’ve not heard of this before?
A leakdown test will tell you what is leaking past the rings, in a percentage value. Air is pumped into a cylinder at Top Dead Center and allowed to set for a period of time, then the pressure is read and compared against the original pressure at the beginning of the test. 2 strokes can have great compression readings, but a poor leakdown value.
I use a compression test more as a diagnostic test, and leakdown as a truer performance test.