My response is going to be so long and all over the place, it should be its own thread.
Let's start with an exercise.
We all toss the ball when we serve right?
Don't move the racket.
What spin happens when the ball falls on to a nearly vertical racket?
What spin happens when the ball falls on to a 45 degree angle racket?
What spin happens when the ball falls on to a flat racket?
You learn the toss itself makes some topspin.
The more vertical racket angle, the more topspin.
Now do another exercise.
You can move the racket, but don't change the angles.
How do you cancel out this topspin with a nearly vertical racket?
How do you cancel out this topspin with a 45 degree angle racket?
You learn that it takes some forward and downward motion to simply cancel out the topspin.
The more vertical racket angle, the harder to cancel the topspin.
As for making backspin, you have to do even more work. Flatter racket angle, faster forward and/or downward motion.
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backspin is the hardest spin to serve. topspin is the easiest spin to serve.
the strategy is to looks for signs of backspin and if there are little to no signs, it is topspin or nospin.
also, i will ignore sidespin since there's no question about that.
the sign of backspin is
quickly brushing the ball forward/downward while touching the
bottom half of the ball.
- quickly
- brushing the ball forward/downward
- bottom half
It is pretty easy to see if they are touching the bottom half or top half of the ball. that is just racket angle.
Serving while touching the top half of the ball is easy to read. It is topspin.
It is also pretty easy to see how fast they touch the ball.
More importantly, slow touch means they cannot override the influence of the toss, which means topspin or no spin.
The tricky part is seeing if they brush forward or brush sideways or both.
if they brush to the side, they aren't overriding the toss: it's topspin.
if they brush forward, you need to guess how much they are overriding the toss.