I understand that you think there is a high pressure at the exhaust valve during overlap, but there is actually the opposite. It may help to visualize the exhaust scavenging that is going on as the exhaust valve is on its closing ramp. The high pressure spike has pulsed down the header tube creating a low pressure area behind it. The old objects in motion tend to stay in motion theory helps draw the intake charge into the cylinder, and a small portion of the intake charge goes out with the tail end of the exhaust charge. The exhaust is always drawing small amounts of the intake charge down the header tube just as the exhaust valve closes on high overlap cams.