That part of the waist isn’t affected. The upper portion of the waistline had the vertices moved down slightly and weight up more on the pelvis to smooth the rotation, then the pelvis is rotated forward in the animations. The UV’s for the top of the beltline are lowered, and then adjusted to push the inner-thigh area over the leg.
That ‘lower’ part of the beltline is generally unaffected.
More work could be done to get the top of the beltline to not scrunch quite a much, this was just a 20 minute rough-draft thing.