🧠It's In the Brain
When you use your quads (thigh muscle on the front) to bring your leg closer to your chest you are stretching your hamstrings (back of the leg). This reminds your brain that there is a relationship between the two muscle groups and therefore allows the back of the leg to release easier. Ultimately, this changes the setting in your nervous system for better, more functional, relaxed, easier on your joints movement.
Stretching and strengthening are key to healing and preventing back pain By Harvard Health Publications. [read]