I'm betting you have seen races that involve hit groups. Hit Groups are basically a way to divide the player up so you can determine which part of the body was hit.

Code like this:
or
are the most common

The first one says "if the hit group does not equal 1(the head) do this"
The second one says "if the hit group is equal to 1(the head) do this"
This generally is used for races like zombies where you must shoot them in the head to kill them. The only hitgroup we really ever see is 1 to distinguish whether it was a headshot or not. Well here is a nice diagram to let you know that hit group 1 is not the only one out there.

http://www.eventscripts.com/pages/Hit_Groups

1 = Head
2 = Upper Body (Heart, Lungs, Rib Cage, etc.)
3 = Lower Body (Gut, Stomach, etc.)
4= Left Arm (includes Left Upper Arm, Left Forearm, and Left Hand)
5= Right Arm (includes Right Upper Arm, Right Forearm, and Right Hand)
6= Left Leg (includes Left Thigh, Left Calf, and Left Foot)
7= Right Leg (includes Right Thigh, Right Calf, and Right Foot)

Those are Hit Groups...now someone go make a race where to die you have to be shot in the arm!

then god help us all!!! 

Hitboxes remain the same, unless you manually remove some hitboxes.
For example, you can add a head to a create model