Overlap actually has to do with inertia of the flow of the fuel/air mixture (from my days in school in the 1960s). As fuel is drawn into the cylinder, there is some inertia in its flow. Just as the piston starts its compression stroke (a few degrees after bottom dead center), the intake valve is allowed to close (camshaft grind of the cam lobe determines when that occurs). This allows just a little more fuel/air mixture into the cylinder, giving a few more HP points.
It's similar on the exhaust stroke. Burned gasses have inertia as they flow out the exhaust port. So the exhaust valve stays open for a few degrees after the piston reaches top dead center of the exhaust stroke as determined by the camshaft grind of the cam lobe