Physical maturity for boosters?

I am trying to make a note on my facebook about why not to put a 3 year old in a booster, but I cannot remember what the physical part of it was, or if I was just getting stuff confused. I know they have to be mentally mature and I know there is a risk of submarining, but I can't remember why there is that risk.
I remember reading that it has to do with the pelvis?

Also I am trying to find something that I read a while back about how in a crash there are really three "crashes" that occur. The car hitting something, the person hitting something in the car and the organs hitting the abdomen, or something to that effect. I don't know if this applies to booster use, but I wanted to write about that in general since it seems like a useful bit of information to share.

Basically I am needing reasons not to booster children too early. I'm not looking for anything that is saying to harness them as long as possible or anything like that though. I have read a lot on here and see that it is not necessary to do that, so I don't want to put out anything saying that. I don't want to freak people out. (As if I don't already with all the rear facing my 4 year old stuff that I have on my FB lol)

I know they have a risk of submarining if they are under 40lbs but another physical part I guess would be their spine not being fully ossified yet.

How does a booster work: A skeleton doesn't reach adult strength until puberty. So a young child's hip bones cannot withstand the forces of a crash as well as adults. The booster acts as artificial hips during a crash. It takes some of the forces, and it acts like a mature iliac crest, holding the lap belt down off the soft abdomen.

There's a risk of submarining and also of head injuries.

The sequence of crashes is:

*Vehicle hits something (another vehicle, tree, etc,)
*Person's body hits seatbelt/harness -- or something else, if unrestrained
*Internal organs hit the body's walls

A vehicle with good crumple zones slows the vehicle gradually, and a properly harnessed/belted person keeps the second "impact" from being an impact, but rather a ride-down as the webbing stretches, which in turn helps the organs to slow down more gradually as well.