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