{HEAD ~ Annulus pressure} then acts against {TAIL ~ the bottom side} of piston 40 and moves the mandrel up and opens the valve 
acts against
{ HEAD ~ Annulus pressure } then acts against { TAIL ~ the bottom side } of piston 40 and moves the mandrel up and opens the valve
HEAD HEAD HEAD HEAD HEAD HEAD OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH