A diffuse axonal injury to the brain is really a nerve injury. In the brain there is the grey matter on the outside and the white matter below that. What happens when the brain moves within the skull in an accident is the grey matter sometimes moves more quickly than the white matter and it shears or tears the connection of these axons. It's this shearing and tearing which creates this diffuse axonal injury.
When this type of injury occurs, the axons provide the communication links between the cells in the brain, and when these are torn or sheared the ability to communicate from one axon area to the other is lost. This can manifest itself in many ways including death. But, more commonly it affects a wide variety of what really makes us who we are. It could affect our memory, our perception, our ability to speak, our ability to perceive, see and judge things that are occurring and a wide variety of other things including motor function.
While the most severe injuries result in death, some of the most profound changes can occur not only to the loved one who has been affected by this injury but those who live with them everyday because of the changes that this nerve damage caused to the personality and the function and what makes that person who they are. This is what makes this type of brain injury so devastating.