In a fight one could get kicked or punched in the head. A strike with good power can not only disorient you for some time but could also knock you out cold. We can not do anything if we are knocked out, however if we are just disoriented we can fight back if we train for it. When we get disoriented we loose our ability to balance, focus as well as gauge the distance between our attacker and ourselves. If we do not fight back when disoriented there is a very good chance that another strike which may not be too hard, will surely knock us out.
The aim of the training is to get the body disoriented and giddy then try to strike the bag as fast as possible.
Do not try this on your own. Always train under supervision of a trainer who is well experienced in what you are training.