There is pressure on both sides of the piston at different stages of the engine's cycle. During the power stroke, high gas pressure on the piston crown combines with the angle of the connecting rod to the cylinder, to produce a high sideways force on the piston. The side of the piston that is exposed to this force is called the "major thrust face", and the opposite side, which sees only the much lower sideways force caused by compression pressure, is called the "minor thrust face". If the pressure in the cylinder were constant through the power stroke, as it is with a steam engine, the thrust force, both minor and major, would be greatest when the angle between the connecting rod and the cylinder axis was greatest. This is the point where the centerline of the connecting rod is exactly at right angles to the line from the center of the crankshaft to the center of the crankpin. However in internal combustion engines the pressure varies widely during both power stroke and compression stroke. Thus the maximum thrust is at a point somewhere between when the pressure is highest, and when the connecting rod angle is greatest.
