Each hole is open for a small portion of the operational cycle to selectively tap an intermediate and discharge {HEAD ~ pressure} to {TAIL ~ the back pressure chamber} 
to
Each hole is open for a small portion of the operational cycle to selectively tap an intermediate and discharge { HEAD ~ pressure } to { TAIL ~ the back pressure chamber }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL