This is an example of hyperextended knees. Don't do this. It's bad. You need to get more on center of her feel with balance dispersed on all four "corners of her feet. Bend at the knees a little and lift up on the quads instead of punching the knees back.
Legs, yes legs! Joan lived without them for 29 years. She recently passed to Paradise. She's dancing with spiritual legs until the resurrection when they shall be restored to her in perfection. A gift of the Atonement of Jesus Christ!