Any time the pressure is less than atmospheric in the crankcase, the reed opens and allows flow, and any time it is atmospheric or above, the reed closes. The charge is then compressed by the down moving piston until the transfer ports open, when it is forced into the area above the piston. The reed has no way of "knowing" whether the decrease in pressure that opened it came from the crankshaft moving one direction or the other. It only senses pressure and opens accordingly.

AV8TOR