1-Elevate your heels on a 2×4 or plate.
2- Stand over a medicine ball
3- Bend over and push into ball as hard as you can, engaging abs
4-Pull your butt down into a squatting position
5- Raise both arms as high as you can and stand up.

This works on your thoracic mobility and your ability to get deep on
your squats.

To progress this just drop your heels further and further until they are flat.