You mean target turrets?
The idea is you zero your scope at whatever range you want, then adjust the turret so the "0" on the turret is aligned to that point. Most I have seen have a small allen screw that locks the turret in place, loosen it up you can spin the turret without moving the reticle.
The idea is that if you screw on an elevation and/or windage adjustment for a shot, it is easy to both make that adjustment, and return to your reference zero when you are done.
In other words, if you need to add 12 minutes of elevation, you spin the knob to the 12, rather than counting 48 clicks. When you are done, you spin it back to 0, rather than counting back 48 clicks.
