Whether you have a predominantly level base swivel, such as the Djokovic backhand above, or the more forward leaning forehand of Federer, in tennis, balance is key. Knowing where your balance is at the point of contact can be difference between a wildly spraying shot and a solidly struck screamer down the line.
Look at how level Djokovic's right thigh is in relation to the ground in picture 2. His left thigh which is hidden is nearly the same level. I believe the idea of being solidly level at all times is a hallmark of Djokovic's game. Note how his butt is well behind his heels in picture two. His back is straight and he is never hunched over in any of these frames. The idea of a level swivel, with the spine as the axis of balance comes to mind.
Even under duress, Djokovic forehand follows the same idea. HIs legs are at the same level, his waist is mostly level, his shoulders also level. Do you followthrough with all these three things level?
Let's take a look at Federer. On a regular speed forehand, he plays with the idea of being balanced and level. He varies his balance and the angle of his spine. Let's look and see how: in FRAME ONE, he is hunched over, loading on the right leg. If you stay on this right leg through the end hunched over, you will spray the shot to the right.
FRAME TWO, he is starting to straighten his spine. In FRAME THREE, he is still leaning back, lifting his right leg all the way. His now straight arm is about to make contact. By the time he gets to contact, his spine is straight and he starts to lean his spine to the left. He swings around the axis of his right leg and spine, lifiting his left foot to continue the rotation. He finishes balanced and leaning to the left, The only frame I see him level COMPLETELY is FRAME FOUR: feet are level, waist is level, shoulders are level.
In the above pic, we see Federer leaning well FORWARDS. He is able to do this, because he jumps off his left leg, not the right leg, loading closed stance style. This is why he is considered a semi traditionalist. He is also VERY OFFENSIVE in his extreme forward balance. His spine is leaning forwards to the max!
You may have an Western style clay court style forehand like Nadal, where the shoulders slope. What I mean is the lead shoulder is always higher than the hitting shoulder from the unit turn onwards to contact. At the FRAME THREE (my count, not the pictured number), he is already dropping the hitting shoulder. He is going up the scale with the forward swing. FRAME FIVE AND SIX: He is really dropping that shoulder and all the weight is on the back foot. Of course, he has to RECOVER THE BALANCE. FRAME EIGHT: he is nearly level with shoulders and waist. FRAME TEN: he is level on everything, feet, waist, shoulders. Geez, that took a long time! lol
So in looking back at these THREE EXAMPLES: you may have a predominantly level and consistent balance throughout the stroke (Djokovic), or you may vary the balance to suit your specialized stroke (Federer/Nadal).