Layers allow you to add successive layers of animation above the original biped animation. This is a powerful way of making global changes to your character animation.
For example, simply add a layer, and rotate the spine forward at any frame, and 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. Layers can be viewed individually, or as a composite of all the animation in all the Layers. Layers behave like a freeform animation: the biped can adopt any position.
Layers allow you to easily adjust raw motion capture data containing keys at every frame. Simply add a layer, and keyframe the biped. You can also use layers to change the global position of the biped in a freeform or footsteps animation by adding a layer and moving the center of mass.
To increment all keys using layers (global offset):
Motion panel 
Layers rollout, click 
(Create Layer) to create a new layer. Enter a name for the layer in the Layer name field.
(Set Key) on the Key Info rollout (you can do this at any frame.)
(Collapse) to collapse the layer.
To increment an interval of keys with an envelope (blended offset):
(Set Key) on the Key Info rollout at the frame where the increment is to occur.
(Snap Set Key) to create keys before and after the incremented range.
(Collapse) to collapse the layer.
To increment an interval of keys without an envelope (nonblended offset):
(Set Key) on the Key Info rollout. Set one key before the frame where the increment starts, and another after the frame where
the increment ends.
(Snap Set Key) to create keys before and after the incremented range to clamp the motion to the offset only occurs in the
range between the two keys.
(Collapse) to collapse the layer.