When 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 {HEAD ~ the vent} 
When 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 { HEAD ~ the vent }
OTH OTH OTH OTH OTH OTH 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 HEAD HEAD HEAD HEAD HEAD HEAD