re: Shooting free throws underhand is superior; However almost no one does it

They've done studies on it. (I forget where)

Underhand does allow you to be off and still make it. It's a forgiving shot.

Normal shot results in on or brick.

There is something about the rotations and soft nature of the underhand...

You will always have a higher % because it is more forgiving to being off the mark.

That said, you'd have to really practice it to find that sweet spot that is foreign to most players.