Many well educated people have written their version of this principle.  Essentially, it is about verbally saying a word when you strike the ball.  The point of it is to give you a singularly focus when getting ready to hit the ball, to eliminate the 'chatter' in your brain.  In this video, the expert, Jeff, uses the word, "YES!" I've also come across,  "bounce, hit" and "split, bounce, hit." The later two examples incorporate saying something when the ball bounces-you figured that out, though :-). 

Article Found Here: Suzanna McGee's Website

​If there is one (one and only) exercise that you should do for the health of your shoulders, this is it.
The modern man’s (and woman’s) shoulders are unfortunately quite dysfunctional. They don’t move in the full range of motion. The chest muscles are too tight and pulling the shoulders forward. The muscles in the upper back and rear shoulders are often too weak.
You may be familiar with the rounded upper back posture, head forward and downward, sunken in chest. Looking your phone or tablet doesn’t help either. Sitting hours behind your computer, or driving in a traffic doesn’t help either.
Wether you play tennis, other sports, are a fitness warrior or just plainly care about your health, you should be taking care of your shoulders. Today’s exercise—shoulder dislocates—is beautifully named because you will feel just like that when you perform it in the beginning. Until you get your shoulders healthy.
Think of gymnasts or ballet dancers, what a beautiful posture they always have. They don’t arch their backs overly, they always keep the core and glutes engaged, but most of all, their shoulders are pulled back and down, in a natural way. Because they are healthy and function in a full range of motion.
When your shoulders are healthy, flexible, and properly aligned, you will have a better posture, less tension in your neck, fewer headaches, a better body alignment and function much more effectively.

 Shoulder DislocatesThis “basic” exercise is a foundation that may seem almost impossible to do at first. If you keep doing it, you will improve rather dramatically and rather quickly. Stay with it, and adopt this exercise as your daily routine, just for a few minutes.

• Have a stick (a broomstick will do, but you can get yourself a stick just for this exercise, and mark lines on it to see how you are improving).
• Grab the stick in front of you, palms facing toward you. Grip it quite wide at first.
• With a smooth motion, lift the stick slowly up, and above your head, and behind your head, whole way down to your glutes.
• Make sure that your ELBOWS ARE ALWAYS straight. If they are bending, grip the stick wider.
• Return gently to the starting position. It is harder to go from back to the front. Make sure you are pulling with the shoulders and NOT BENDING the elbows.
• Perform 5—10 repetitions.
• Keep your core and lower back steady, do not move it.
• As you get better, get the grip slightly closer—inch by inch, over time, you can work yourself up to almost shoulder-width. But don’t rush. This is a long term journey toward your shoulder health.
• If it is impossible to do with the stick, start with a rubber band. It will give a little bit during the hard points. But as soon as you get more flexible, transition back to the stick.
• You will feel some pain and discomfort. On a scale from 1—10 (1 the lowest, 10 the highest), you should not be much higher than 6. If it is that painful (7 or more), get a wider grip and start gently, or go back to the rubber band.
• Only after a few sessions, you will feel really great in your shoulders. Keep working on it.
• When you can do a very narrow grip, very smoothly, you can add an extra weight to the stick. Add 2—5 lbs at first. If it is difficult, start with wider grip and work yourself into a narrower one again.

Suzanna Mcgee's Article on her website both explains the "why" this move is important and the "how" to do it correctly.

Suzanna's Website 
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