[91civichatch2571]

Its just a little rubber hose with threads on one end (they thread into your spark plug hole) then on the other end of it theres a pressure gauge. You crank the engine so it turns over 5 or 6 times and with each stroke of the piston, air is compressed which causes the gauge to raise. If your numbers are low, the air isnt being compressed too well, its escaping somewhere like the rings, a valve thats either stuck open or isnt closing all the way or through the head gasket. The thing about compression testers are that you cant tell where the air is escaping from. Thats where a leakdown tester comes in as Jos4dcivic mentioned. It pinpoints where you are losing air.