When the vent is moved to one side of {HEAD ~ the piston}, a pressure imbalance occurs across {TAIL ~ the piston} forcing it to move in the direction of the vent 
When the vent is moved to one side of { HEAD ~ the piston } , a pressure imbalance occurs across { TAIL ~ the piston } forcing it to move in the direction of the vent
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH