When the exhaust valve just opens there is still about 100PSI in the cylinder. This roars past the exhaust valve at supersonic speed. Although we do not normally feel it, air has mass as does the exhaust. That mass and velocity creates inertia as that slug of exhaust roars down the exhaust pipe. When the exhaust valve closes, this creates a low or negative pressure in the exhaust. I assume these had a check valve in them to prevent exhaust getting out.