I have a pump with a presta fitting on one side and a schrader fitting on the other side. If I leave the pump unattached to anything and work the handle, air squirts out both orifices. But if I put my thumb gently over either hole, the pump pressurizes on the side closed by my thumb and no longer squirts out the still-open side. How does this work?
Note 1: This is a double sided head on a stand pump. There is a selection lever but the effect is there without pushing the lever to either side.
Note 2: There is no need to push a valve into the pump head to see the effect. Blocking a hole by putting a flat cover gently over its outer surface is sufficient.
Note 3: I first noticed this effect when pumping a completely empty tube (outside of a tyre) up from flat. With the valve firmly in the nozzle, and the lever on the correct side, the air insisted on rushing out the wrong (open) side of the pump head. Only when I pressed the tyre rubber almost flat against the inside of the valve, did it build up pressure on the 'right' side and allowed a little air into the tube. That was then sufficient to pressurize the valve so it then pumped all the air out the 'right' side on subsequent strokes.