Myth - To line up the ball on your overheads you should point your finger up at the oncoming ball before you hit it.
Truth - Just another variable to think about that is NOT necessary. If you already have a habit of pointing your finger toward the ball, that is fine no need to change it. Some pros do and some pros do not point their finger. The idea behind pointing your finger at the ball is to help you judge the flight of the ball and line it up for the final hit with your racket. Sounds good, but here is the problem. It does not work! All I have seen from this technique is confusion and the addition of one more thing to think about while learning to hit overheads.
Beginners point at the oncoming ball as it goes right on over their head or even hits them on the head! In their confusion while hitting an overhead beginners try to remember, "I have to point, I have to point!" And they do point - at the wall, at the lights, at the doors, and even at the ground! I remember the first time I noticed a student pointing at the ground when she moved to hit an overhead. At first I could not imagine what she was doing? Maybe distracting her opponent while smashing an overhead past them? - similar to pointing to the sky making everyone look up while you do something else. Then it dawned on me. She was trying to point at the ball as she had been taught. In her confusion to reach the overhead, she suddenly remembered to point, but forgot what she was supposed to point at!
The key to finding where the ball is, as always, is judgment. You must go out and hit many overheads over and over and over again until you can judge where the ball is located. Pointing your finger will not make you judge the ball any better. In fact, it can be a distraction to you! - distracting you from some of the simple things necessary to learn an overhead. Like preparing the racket and swinging at the ball!!! If you have already developed a habit of pointing at the ball, no need to change this habit. Pointing at the ball is not wrong, it is just not necessary to hit a top notch overhead.
If you do not point at the ball, what do you do? Most pros are relaxed allowing their non hitting arm to raise up, but not all of them point at the ball. If there is one common denominator when the pros hit an overhead, it is that the non hitting arm is held up before hitting the ball. Some pros point at the ball, some pros have their elbow up, and some pros have their hand up. Try and stay relaxed and let the non hitting arm find the style that is natural and comfortable for you.