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