Activate this option only when you import older (pre-MotionBuilder 5.5) FBX files that contain an animated joint hierarchy, for example, an animated character.

This option lets you assign the rotation transformation of the null (or joints) elements in the hierarchy that are used as pre- and post-rotation to the joint orient and the rotate axis of the original node.

The pre-rotation and post-rotation nodes are then deleted. Older files created with the Export Pre/Post Rotation as Nulls option are merged back to the original Maya setup.