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