basically the green sphere should cover only the area you want to move a hundred percent with the bone, and the red sphere should overlap the green sphere some and cover only the area you want to blend between a hundred percent to zero percent. thats a pretty simplistic explination but it will give you an idea of how to go about working. the best way to learn is to study existing rigs and see how they are set up, and see what adjusting them a little bit does to the joint.