Here you go...straight from Leupold.
"The rear ring is secured to the base by two screws, one on each side of the base. By loosening the tension on one screw and increasing the tension on the opposite screw the scope can be adjusted for windage(right/left). Move the back end of the scope in the direction you want the bullet impact to move. For example, if you want to move the point of impact to the right, loosen the right windage screw and tighten the left screw. By using this feature the shooter can leave the scope's internal windage adjustment at the center of travel while putting the majority of the windage correction into the mount base. This may be necessary when the mount holes in the rifle's receiver are not in true alignment with the barrel. The windage adjustment feature of the STD mount system is also a benefit to the long range target shooter. By keeping the internal windage adjustment at the center of travel the shooter will maintain the full range of adjustment in the elevation axis."