The exposed area on the flat part of the head in the cylinder on the intake and exhaust side. The pinning actually deflects the head outward .001 when done. Then they machine the head flat again. After they are done the weak combustion chamber area that usually deflects in 130+mph cars doesnt anymore. My friend melted a cylinder and hammered the rod bearings in an engine of his but the gaskets did not show any signs of compression leaking over them. Gaskets used were cometic and believe it or not there was no re-torque done. Car went as fast as 128mph at 3700+lbs. His heads were pinned.