Currently the animation validator on the simulator fails to recognize "GROUND" as a special constraint, reporting the following error:
> Animation file avatar_sit_ground_constrained references unknown joints: GROUND
This constraint is used in official animations such as avatar_sit_ground_constrained.bvh (See https://wiki.secondlife.com/wiki/Internal_Animations ) with the UUID
1a2bd58e-87ff-0df8-0b4c-53e047b0bb6e
.
This constraint allows the viewer to automatically IK the feet to the ground surface by raycasting, allowing the avatar to keep it's feet on the ground on uneven surfaces or terrain.
If you need a test animation, I can provide one.