The Base Frame series of properties defines the reference point to calculate the motion alpha of the Twist Frame. In the Bone property, you can select the bone to use as the Base Frame from the characters skeleton. In the Axis property, you can select what axes of motion to calculate motion. A value of 0 will not calculate motion, a value of 1 will calculate positive motion, and a value of -1 will calculate negative motion. You can also enable the In Local Space property to calculate motion in local space, or disable In Local Space to calculate motion in world space.