WHAT IS A COMPRESSION TEST??????????????????

A COMPRESSION TEST IS A TEST DONE THRU THE SPARK PLUG BASICLY, WE PUT A TESTER THRU THE PLACE WHERE THE SPARK PLUGS NORMALLY GO AND YOU CRANK THE ENGINE AND A EXPERIENCED TECH WILL UNDERSTAND THE READING FOR EVERYTHING FROM THE VALVES TO THE PISTON RINGS. THIS IS DONE WITHOUT TEARING THE ENGINE COMPLETELY APART AND USING A FEELER GAUGE TO CHECK THE ENGINE WHICH COULD COST A SUBSTANTIAL AMOUNT MORE THEN THE COMPRESSION TEST...