I have to use my arm muscles to keep from whacking into the window frame. If properly shaped, the flying hand can smoothly and effortlessly lift and steer your arm. The key is a good airfoil shape. Wortman type airfoils are very efficient, but require extrordinarily precise finger positioning. The hand shown above is using simpler and more forgiving finger positioning.
Slotted airfoils are beyond the beginner's grasp.
At speeds of 50-60 mph, the hand should be extended out the window. The angle of attack can be rotated so that the lift forces acting on the hand just balance the weight of the arm. Now relax and enjoy the journey. The pitch angle must be steadily increased as the car slows. At the first sign of buffeting, the hand can be deeply cupped to increase the lift and drag temporarily.