When you work with a biped, you use Figure mode to fit the biped to the mesh or mesh objects that represent your character. You should have Figure mode turned on when you attach the mesh to the biped with Physique. Figure mode is also used to scale a biped that has a mesh attached to it, to make biped "fit" adjustments after Physique is applied, and to correct posture for motion files that need a global posture change.
When Figure mode is turned on, the biped jumps from its animated position to its Figure mode pose. Animation is preserved when you exit Figure mode.
The parameters on the Structure rollout are active only in Figure mode, and at creation time.