does anyone know why there is such stepwise improvement seemingly in all tbi cases? either spontaneous within the first week(s) or month(s). if it is regeneration of tissue.. that doesn't really make sense does it?

There is some evidence that the brain has a system that tries to recover from injury but after a while, it may recognize that certain parts of the brain are not able to recover. At this point, it sends out die signals those cells die and are absorbed. Now, the brain has to learn to work around these broken areas, if it is possible. My neuro stated this in a 'recovery reserve' way. For some, this recovery reserve can take over. For others, this recovery reserve has already been used up. This would fit with the Multiple Concussion Syndrome process when after a number of concussions, recovery is haphazard and prolonged with some symptoms remaining.

The other issue to consider is that some concussions are just in the brain. others include upper neck injuries that impact how the brain recovers. Until those upper neck injuries are treated and healed, the brain continues to struggle.

There is what is called a brain stem concussion. This can have some difficult symptoms with some dysfunctions that last.

i understand, however, how does that explain the spontaneity effect? ie if there is a system to recover from a concussion that the brain uses then it must not be by somehow "healing" the damaged area but by forming neural connections around it? and once they are formed you feel recovered? the reserve presumably means spare neurons?

The spontaneous recovery could be that the injury did not have any upper neck injury to complicate the brain's recover and/or the brain was injured in areas that are better able to heal. Since those spontaneous recoveries usually have minimal medical intervention, there is not a big chance to collect data and understand the issues.