Excessive lumbar extension in the squat is not a good thing. The FIRST movement you make when squatting is pushing the hips back (not breaking at the knees). However, keeping the hips and spine neutral is pretty important. Notice in the picture on the right how keeping the chest up too high will result in anterior pelvic tilt and excessive hyperextension of the lumbar spine. Over time, this =snap city. If you often suffer from low back pain when you squat, you’re probably in this camp.
The “chest up” cue, thus, may not be optimal for lifters who tend to suffer from APT. It’s important to understand each individual lifter’s anatomy and movement patterns before blindly assigning cues that may not work for them. If you’re the person on the right, try bringing your chest DOWN as you sit your hips back as opposed to forcing the chest up.