It is a bullet drop compensator... effectively a 'huntingerized' elevation target turret. you set it up (obviously) to your ammo and can have a zero stop then 3 colour coded ranges for instant dial in... no counting clicks etc... so you may have your zero stop at 100yds, green at 200, yellow at 250 and red at 300 or whatever you so desire... no more gueswork or texas holdovers, if the beast is outside your PBR then just dial in the relevant colour for the distance you need and take the shot..