The drop test works but it only tests and one impact velocity. Tieffenbacher showed that the normal COR changes depending on impact speed.
You seem not everyone knows what I think is obvious.
Normal is like perpendicular only perpendicular only applies to a line. Normal is similar but it applies to a plane. A flag pole is normal to the ground.
Tangential would be any motion or force along that plane that is normal to the flag pole. Now since the earth is round, a plane that is is normal to the flag pole would leave the ground as the distance between the point on the plane and the flag pole increased.
So when the ball hits the paddle, the two exert a force on each other. The force the paddle exerts on the ball can be broken down into a normal force and a tangential force. So there is a tangential and normal COR. The normal COR affects speed and the tangential COR affect spin. The COR number are between 0 and 1 but for most modern rubbers they are probably between 0.6 and 0,7. A normal COR of 0.7 means that if the ball hits the paddle at a speed relative to the paddle of10 m/s it will have a normal rebound speed of 7 m/s relative to the paddle but if you are moving the paddle towards the ball at 5 m/s the ball will be moving at 12 m/s relative to the ground/ table.
If the paddle has a tangential COR of 0.5 and the paddle is moving horizontally at 10 m/s is horizontal. The ball will travel forwards at 5 m/s or or less. Much depends on the force of impact. The tangent COR changes depending on the impact speed. Obviously, to me anyway, that if the ball only drops from 1 cm that the ball will not impact with enough force to to grip and propel the ball forwards with back spin.
I have been posting the Tiffenbacher document for years and now realizing the few understood any of it.
https://deltamotion.com/peter/TT/199408014 - Tiefenbacher - Impact.pdf
Tieffenbacher is German so he used the term Epar and Tpar for Normal COR and Tangentia GOR.
Pay attention to the second small paragraph.