When you bend forwards without thinking, normally both your hips and your lower back will move.
Many people with lower back pain have too much movement taking place in the lower back, and not enough at the hips. This can lead to increased strain on some of the structures in the lower back, and eventually to injury and pain.
Sometimes the problem is a lack of flexibility at the hips – tight hamstrings for example often mean that the lower back has to move more to compensate for the restriction. But often, part of the problem is due to motor control factors – basically, bad habits.
Hip hinging is a good way to begin training your brain to maintain a stable spine. For athletes who have trouble separating hip and back movement, I always start with this exercise before moving on to loaded exercises such as deadlifts. Many find it harder than they expected to keep the spine straight and ensure that the bar / pole stays in contact throughout the movement.