Originally Posted by 10isfreak
The angle with the normal can be 10 to 20 degrees, so with the ground, 70 to 80 degrees. You are looking at the swing path of the eventual point of impact.
Well, the horizontal line in the drawing is parallel to the ground. If that model turns out to be true, following the advice of many coaches who ask you to brush up for more spin and meet the ball with a square (vertical) face would result in sending the ball higher (the vertical swing path) and being left with too little spin (the vertical racket face).
To my sense, it's what seems to happen to many amateurs.