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