Each biped added to the Motion Mixer is automatically assigned a balance track. You don't place motion clips on this type of track as you do with transition tracks and layer tracks. The weight curve on the balance track is the only adjustable parameter.
By default, the Motion Mixer compensates for differences in upper and lower body motion that might cause the biped to go off balance over the course of the animation. It accomplishes this by changing the COM, pelvis and spine animation.
When the weight curve across the balance track is set to 1.0 (the default), balance compensation is enabled for the entire animation. You can adjust nodes on the weight curve to disable balance compensation over all or part of the animation. See Adjusting Biped Balance in the Mixer.