It seems almost instinctive to keep your head down when accelerating as many sprinters do now-a-days. Does cervical flexion contribute to posterior pelvic tilt thus generating more power to the glutes and hamstrings. Inversely, does the head coming back in extension aid in recruiting the low back, creating anterior pelvic tilt and working as a hip extender as the glutes and hamstrings fatigue late in a race?
Maybe not the cervical flexion directly but I think the benefit is in cueing the entire body to adopt efficient mechanics rather than being causal in itself.