Sometimes a bad bucker/rearer needs to have his head tied in order for him to learn that if he blows up like that... he's going to loose his balance and fall.... yes its a hard lesson but if a horse is dangerous and nothing else works for teaching him/her not to buck or rear... then it will sometimes work as a last resort.
My friend had a bucker. She trained the filly since she was 3 and around her 4th year for some reason she went into bucking fits. (We think it was her way of testing the rider... nothing was setting it off. Her saddle/tack fit fine). The only way she could break this stupid habit the filly was setting herself into was to make her fall over when she went into them. So tieing her head down and having her fall over did the trick. It only took two times for her to fall over for her to not try it again. Now she rides like any other broke horse.