RE>Leak down test instrument

[Johnston, James W]

>>Someone gave you the wrong answer before...

Correct me if I am wrong, for I am surely not a professional, but what I
know of leakdown tests in my experience is similar to a compression test
where you crank the engine and watch the pressure in the cylinder slowly
dissapate to see how well it holds the pressure...why would you need to
pressurize it externally with compressed air and that special 'leakdown
tester'?  wouldnt a compression tester be sufficient?

thanks for clearing things up..
-j