The Bone Driven Controller allows a "Driver" bone to dynamically affect the motion of one or more "Driven" bones. This is useful for characters that have accessories attached as you can use this to avoid geometry from intersecting during an animation, even when a lot of blending is being used.
You can either set the "Driver" value directly with a Multiplier, remap it to a whole new range, or use a Curve asset to drive the motion. Using a Curve is usually the best approach as it lest you define the response naturally and interactively tweak points/tangents while seeing the changes reflected in real-time.
In the example below, the attached accessory (green) has no authored animations, and is being driven in two axes as a function of the character's thigh bone. This is all calculated at runtime, so a blend of animations ends up working quite well here, even without hand-authored tweaking.