Only the upper body rotates.
Animate the Balance Factor:
1 Select any part of the other biped.
2 In the Track Selection rollout, select Body Horizontal.
3 In Key Info rollout, click Set Key.
Now Balance Factor is available.
4 Change Balance Factor to 2.
5 Select Bip02 Spine. Rotate this spine.
Creating the Illusion of Weight | 959