What is does is when your cyl. fills with pressure the gauge reads it just like a compression tester but this one you have a line hooked to it applying air pressure back into the cyl so that if any thing is leaking past the valves, rings, or out the head gasket it will read it. Kinda like a compression tester but it dont just pull the highest reading and hold the gauge there.

 

and also you will hear a whistling noise coming from the engine (or crank case ventilation unit) if its anything piston/rings related problem

 

Have you tried to put a little oil into the cylinder?That is the easy wet comp test.