You were thinking of a compression test. A compression test just tells you how much compression any particular cylinder is making. This is useful, but not a good as a leakdown test. A leakdown test actually pressurizes each cylinder while you monitor the pressure and listen for leaks. If a leak is detected, you can pinpoint if it is leaking into the intake, exhaust, cooling passages, or crankcase. A leakdown test gives you much more and more accurate information than a simple compression test. See here:

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