You can find this exercise on page 158 of the book Synergistic Golf.
Exercise Type: DYNAMIC STRETCH
Exercise Category: FLEXIBILITY, HEAD - NECK
Stand or sit keeping chin slightly elevated and think about a long neck. Inhale to prepare. On exhale, drop chin toward chest allowing back of neck to lengthen. Inhale back to center. Exhale and allow head to drop back lengthening front of neck. Open mouth to release neck flexors. Repeat slow and controlled 10x.