Pair Valve is a one way reed block. Draws air from the airbox but not the reverse. TdubAt last, a straight forward and correct answer by 'Cozey'.
PAIR REMOVAL is only for tuning purposes to enable accurate readings on the dyno etc. and avoid polluting inlet air . It is listed in the HRC Tuning Manual where hose block off bungs are the only parts used. Everything else is left connected probably for WSB Regs ie weight etc