Upper back begins to round toward the bottom. Keep it tight by trying to keep your "chest up" even as you are bent over at the bottom.
In addition to chest up, look slightly farther forward. ~1.5 to 2m. Pick a spot and keep your eyes focused there
Heels come up and you move onto the ball of your foot on some reps. Slow down a bit and keep awareness of your balance, should be over mid-foot, throughout the entire rep