When you blend from IK to FK, you rarely want the FK Rotations at their default values, which would make the limb point straight down. By clicking the Match FK > IK button, Limber will work out what the FK Rotation properties need to be in order to match your current IK pose, and set their values accordingly. Similarly, if you are animating in FK, you can click Match IK > FK and Limber will set your end controller’s position property to match your current FK pose. Matching IK to FK can also alter the Clockwise property, since FK Rotations can be either clockwise or anti-clockwise. In this case, Limber will add a keyframe to the Clockwise property if it is already keyframed, or simply set it's value if it is not keyframed.