The crankcase is never truly cut off from the intake area either... When the piston is moving up (sucking in new charge) the skirt is above the intake windows but as the piston moves downward (to compress what's in the crankcase) the two piston intake windows pass behind the cylinder intake windows. As they line up, the charge is allowed to pass back through the intake windows into the intake area. Some argue that's the reason a intake boost port works even at low RPM. It allows an alternate path for the charge to pass into the upper part of the cylinder. Instead of charge moving straight up and through the transfers it's also allowed to pass through the intake windows and up through the boost port. Once the engine is in the upper RPM ranges the boost port is force charged by intake velocity, no longer allowing crankcase charge to pass through it.