This Hunter probably has a Martin Baker Mk 4 seat in it (although some single seaters had earlier seats), and this seat has a 80 ft/sec gun which is at the limit for damaging your back (rocket seats have 60 ft/sec guns). When you bang out there is a probability that your spine will not be straight as it s difficult to be sure that you are sitting correctly. If your spine is straight all the vertebra will be parallel and you should escape any injury. However, if your back is slightly bent then the vertebra are likely to collide with each other and fracture - depending on the bend this will be the front or the back of the vertebra bones that will fracture, and in the day you might expect several weeks flat on your back to allow the vertebra to re knit..