The forearm has close to 180 degrees of motion whether the arm is twisted palm up or palm down.
The twisting of the palm is rather complex in the body mechanics involved so body links simplifies it. You'll notice you see no wrist rotation in the side profile. Sometimes you'll see a 180 degree rotation in the front profile. In general body links does not worry about that motion.
Body links is a simplified way of placing the body part in the general area needed. It is not important how the hand grasps the object just that the hand is in the area to grasp. Palm up, down or sideways doesn't concern us in these calculations.
With these two links you can determine the needed placement of your hand or your object from the elbow out. When the hand interacts with your object you can also calculate stress on the joints based on the links and the angles those links are held at. Instead of calculus you can now use trigonometry.
