A vehicle's engine contains a number of cylindrical cavities that are bored into the engine block. Each of these engine cylinders houses a piston that moves up and down to spin the crankshaft, much the same way someone might pedal a bicycle. Inside the cylinder is where engine combustion takes place to move the pistons up and down. The cylinders are sealed airtight, with valves that open and close to allow air and fuel to enter the cylinder, and to allow exhaust gasses to escape. When the valves are all closed, the cylinder builds up either compression or vacuum (depending on which direction the piston is moving). In both cases it is necessary for the cylinder to be sealed. If one or more of the cylinders is not sealing properly, the engine will not run properly. A compression test is aimed at determining if a cylinder has low compression - a sign that it has an inadequate seal.

To generate power, an engine utilizes the exhaust gasses created in the combustion process as a source of power to move the pistons up and down inside the cylinders. The engine in a car is known as a "four-stroke" (or "four-cycle") engine that relies on four distinct piston strokes (intake, compression, power, exhaust) to complete one full operating cycle. During the compression and power strokes, the intake and exhaust valves are closed and the cylinder is sealed airtight.