I am learning about Root animation and here come two terms, one is root transform and Body transform. As name suggests that body transform is the character body but what is root transform? and what is the Root transform rotation based upon body orientation, Feet, center of mass or original.
Taken from the link you've provided:
The Body Transform is the mass center of the character.
So, when you animate a character slipping and falling to the ground, if you don't modify their Body Transform and only change the Root Transform, the animation will be played and the character will fall on the ground. The problem is, the body's center of mass is a feet or two above its body now, and if it interacts with and other physics object, it will act as if it's body mass is in the air.
Now, in this situation, the car will still hit the body, and the body will still react, but it will spin around its center of mass, because of the force applied on it:
Now, we should see the body spin around its center of mass. I haven't tried this exact scenario, but it should be how it works. Just try it out in Unity (I will do the same and update the answer with results as soon as possible).