A common thing coaches tell their students is that to hit a slice, toss the ball further to the right, to hit it flat, it’s more in the middle, and to hit a kick serve, toss it behind you and arch your back.
I really disagree with this common teaching tip. Yes, if your toss is extreme enough one way or another, you will be forced to hit the ball with a certain sidespin. But at higher levels of play, this can be a death-sentence for your serve. What makes the best servers in the world so great is that they can toss the ball exactly the same way every time, and do the same motion, but direct the ball any number of ways. In other words, they are totally unpredictable until they hit the ball.
If I am playing someone who tosses the ball behind their head, I instantly KNOW that they are not going to hit a slice serve. They also will not be hitting a flat serve out wide because they physically cannot come through that way with their shoulder. If I see them toss the ball out to their right side, I instantly know even before they hit the ball that they will likely hit it with slice, and towards my forehand side. As a returner, I can instantly adjust my court position and neutralize every single one of their serves.
Disregard my slop service motion here and just look at the toss locations – If you opponent tosses into the yellow area or further, it’ll be a slice. In the red area or further, it’ll be a kick serve (ouch! my back!), and if it’s in the green, it’ll be unpredictable.
If the difference in the placement of the ball toss is subtle (less than 2 inches), then it’s probably fine, but if your toss varies greatly depending on if you are trying to hit it slice, flat, or kick, then it’s counter productive. You have to use a different motion for each serve, you can hurt your back and shoulder, and your opponent will see it coming a mile away. Practice hitting all your serves with the same ball toss.