Lesson 1: Flexibility is determined by joint position (posture).
Now stand up again and this time squeeze your shoulder blades together and hold them. Now bend over to touch your toes. Notice the difference? You were able to reach lower and you felt less tight in your hamstrings.
Makes you think does it? You increased your flexibility by changing joint position again, but this time you changed a joint far away from the area you were stretching.
Lesson 2: The body is a unit and flexibility of one muscle group can be influenced by joint positions (posture) throughout the body.
Now while sitting on the floor, bend your knees so your feet are flat on the floor. Rest your hands on top of your knees and lean your upper body backwards until your arms are straight and your upper body is leaned back about 45 degrees. In this position, roll your hips forward to arch your back as much as possible. Hold this position for 30-60 seconds. You will probably feel a lot of work in your hip flexor area.
Now straighten your legs out in front of you again and repeat the sit and reach test. Notice how you are able to reach further with less effort now.
Lesson 3: Increasing the contractile ability of one muscle group increased the flexibility of the opposing muscle group.