However hyper extension of the neck happens even without a mirror, so as long as you are mindful of it, no problem. (rugbystrengthcoach.com)
Hyper extension in the thoracic spine exerts pressure on posterior mediastinum - a part of the thorax which when pressured creates an increase in the sympathetic tone of the autonomic nervous system (responsible for the fight or flight mechanism)- this is not a good thing. (rugbystrengthcoach.com)
Athletes typically go into thoracic hyper extension to utilise the accessory respiratory muscles (intercostals, sternocleidomastoids etc.) to make up for their lack of ability to properly use the diaphragm during breathing. (rugbystrengthcoach.com)