Your engine's crank case has pressure and vacuum inside depending on if the piston is going up or down.

The reed valve is a directional one-way valve that opens when there is a vacuum inside the crankcase and closes when there is pressure.

When there is pressure inside the crank case and the transfer ports to the combustion chamber are opened by the piston - the pressure in the crankcase escapes to the combustion chamber.

DLE reed valve blocks sometimes warp slightly causing the the reed valves to not seal very well.