When you animate a biped or other object, you can add layers of animation above the original animation. This is a powerful way of making global changes to your animation. For example, if you add a layer to a biped animation and rotate the spine forward at any frame, a run cycle becomes a crouched run. The original biped motion is kept intact and can be viewed by switching back to the original layer.
You can view layers individually or as a composite of all the animation in all the layers. Layers with bipeds behave like a freeform animation; the object can adopt any position.
Layers used in conjunction with Biped let you easily adjust raw motion-capture data, which contains keys at every frame. You do this by adding a layer and keyframing the biped.
Layer controls for Biped are on the Layers rollout. For other objects, use the Animation Layers toolbar.