This lever is used to adjust the clamp on the Arca Spec mounts as well as the pan/swivel and cant motion on the G2 bipods.
For instructions on how to swap and maneuver the lever, watch this video.
DO NOT TIGHTEN WITH A TOOL OR ALLEN WRENCH! This lever is adjustable. If you hit a stop and the tension is not where you desire, pull the lever toward you, reposition, and tighten.