well,, that lot confussed me, so here's my take in laymans english
you already know what distance your crosshairs are set on, say 40yds for argument sake, you look through the scope, with the parellex set high, say 150yds, the picture through the scope will be blurry as hell, slowly wind in back, whilst looking through scope, as the picture becomes spot on clear, look at front of scope, and the yardage its at, now it may so 50yds, so you know you need to aim a bit higher with your crosshairs,
in all honesty Ive never found them of much use on air rifles, more usefull on the rimfires