When the piston moves, {HEAD ~ the power transfer shaft moves} in or out as {TAIL ~ the case} may be 
When the piston moves , { HEAD ~ the power transfer shaft moves } in or out as { TAIL ~ the case } may be
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH