Lie supine on mat.
Push head into mat and hold position. Relax and repeat.
This exercise may be prescribed for functional forward head posture since it strengthens Splenius without neck flexion. The stretch that occurs during complete neck flexion, may exacerbate forward head posture, particularly if neck flexion is held under load. Neck flexion normally occurs at end of eccentric portion of dynamic Splenius exercises.
Neck retraction primarily consists of Cervical Flexion (spine) accompanied by subtle Thoracic Flexion (spine) and Neck Flexion (atlanto-occipital joint). Cervical spine has natural lordotic curvature (see diagram of spine). Much of the retraction motion is actually flattening or slight reversing the normal lordosis. The deep neck flexors perform majority of movement. Slight neck Flexion at the atlanto-occipital joint also occurs to maintain forward head positioning.
Also see similar exercise Neck Retraction Stretch.