You have to know about cylinder pressure and how cylinder pressures change or behave during the compression and power strokes of a 4 stroke engine.
The way to locate the exact position of a piston along it's travels up and down the cylinder is by using the crankshaft degrees, as a reference point:
When the piston is at the very top (top dead center, TDC), it's at 0 crankshaft degrees. When it has traveled down and reached the very bottom , it has gone 180 degrees. When it reaches the top again, it has traveled 360 crankshaft degrees. That was 1 complete revolution of the crankshaft.
Here is a graph showing you what the pressure (Y-axis) is inside a cylinder and how that pressure behaves, as your crankshaft turns and the piston is going up and down, during the compression stroke and power stroke.