The cervical spine consisits of seven vertebra numbered C1-C7. The back of the head or skull is called the 'occiput' and is numbered C0. The occiput sits on the first vertebra of the cervical spine (C1), which is called the 'atlas'. The juncture between these two bones is called the 'occipito-atlanto joint'.
The second cervical vetebra (C2), is called the 'axis'. The junction between atlas (C1) and the axis (C2) form the 'atlanto axial joint'. The axis is unlike the other cervical vertebrae, as it has a fingerlike projection called the 'dens' that protrudes through C1, so the atlas can rotate around it somewhat like an axle of a wheel (though the movement is far from a complete rotation).
The bones C0-C2 include the aforementioned joints and ligaments, but unlike the other cervical vertbrae have no discs. The lower cervical have intervertebral discs that do not allow for lateral flexion.
As a coach you can use this imformation to help identify when an athlete is maximally contracting during an upper trap movement. Since the occipito-atlanto and atlanto axial joints do not have discs they have the ability to move to opposite sides of lower segments.
The best movement for the upper trap is a one arm shrug. When the athlete reaches peak contraction the upper cervical vertebra can and will pivot on the dens and the athletes head will turn away from the elevating shoulder shrug action. The muscles of the upper cervical region at C2 have an individual specialized arrangement and when fatigue occurs and the upper trap is truly targeted the head will turn without any coaching. If the head does not turn the athletes form is incorrect or there is not enough effort. You only need to observe, asking the athlete to tilt their head would be a poor coaching cue, as the head tilt occurs naturally.
Training the Upper Trap on the Pendulum 5 Way Neck