Since defensive players are supposed to see both their man and the ball at all times, and since there are no pockets in most basketball pants, a good place to put your hands is out to the sides, one hand pointing to your man and the other hand to the ball.
Not that you have to have your index finger on each hand pointing like Uncle Sam on an Army poster, but why not have one hand keeping track of the ball while the other keeps track of your man? Why not have your hands up at the shoulders where they are ready to touch a ball if it comes? With hands up and open fingers pointing to the two things you are wanting to keep in your view, you are constantly reminded of your objective: to keep both in view at all times.
Though most coaches do not demand that you point to both the ball and your man, there is not much excuse for doing anything else with your hands. Keep those hands out and up and point. The effort at pointing constantly will almost certainly tune your concentration more finely and help you to play better defense.
From Dick’s book Stuff