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