Yeah, I figured that would have happened a good ways back.

What we call pinch is where your OS engine is different than your car. Your car has cylinders with straight walls. It has the same diameter at the top and bottom. The compression comes from the rings as they squeeze against the cylinders. Your engine doesn't have rings, so it has to make compression some other way. This is done by making the cylinder slightly tighter at the top. The piston hits this tighter area as it comes up and this creates compression.

The compression is necessary to squeeze the fuel/air mixture for being burned.