The piston and its half of the connecting rod stops twice per crankshaft revolution, even though the crankshaft continues to turn. This means the piston and top of the rod also start back up twice. This stopping and starting imposes stresses on all three of the parts, stresses that increase with crankshaft rpm. To reduce these loads, the piston is mounted to the connecting rod slightly offset. This causes the piston to reach top dead center at a different time than the connecting rod, effectively spreading the shock loading over a greater number of crankshaft degrees. In short, the real reason for piston pin offset is that it softens reciprocal loading, permitting lighter more power-efficient parts to be used, and the engines to be capable of higher rpm.

However, there is another phenomenon at work also, a kind of side benefit. Because the connecting rod spends most of its time in the engine at an angle, the piston engine has what is called minor and major thrust. Major thrust refers to the downward-stroking piston's force against the cylinder wall during combustion, due to the rod being angled in that direction. Minor thrust is the piston's thrust against the opposite cylinder wall during compression, because the rod's angle is opposite also. These thrust forces push the piston firmly against the cylinder wall. The important thing is that at TDC, they flip-flop. Major thrust turns into minor thrust, and visa-versa. In older engines, this flip-flop caused the piston smack the cylinder, resulting in a noise. Fortunately, the piston pin offset in today's engines, besides reducing inertia stresses, does two things that reduce this noise. First, because the piston is mounted off center, the transition from major to minor thrust is less sudden. There is less impact. Second, instead of a sudden lateral shift, the piston actually rolls from major to minor thrust. That is, the piston shifts first at the skirt, then gradually the rest of the piston makes contact, instead of all of the piston at once.

To summarize, piston pin offset is the manufacturer's way of reducing stress on reciprocating parts. It permits these parts to be lighter, which results in more efficient manufacture and less power loss in the engine, as well as higher rpm. A complementary result of piston pin offset is reduced piston slap due to the more gradual shift from major to minor thrust.