This is a great exercise to strengthen the deep cervical neck flexors. These muscles are very important in stabilizing the cervical spine. Often with whiplash accidents they are damaged. Forward head posture has a tendency to inhibit these muscles and increasing shear loads on the cervical spine.
As you let your head dangle, create a double chin. Hold the double chin and lift your head up about 2-4 inches. Make sure you hold the double chin. You should feel it on the front of your neck. If you feel it on the back, you lost the double chin and have shifted the workload to the cervical extensors. You don't want that. Try incorporating 3x15 reps each day.