Originally Posted by bhupaes
The angle ϕ being non-zero is not the reason for sidespin. Just think, if what you are saying is true, there will be sidespin whenever contact is made with a laid back wrist - and this is definitely not so. On the other hand, even with ϕ=0, it is possible to have sidespin if the path of the racquet head at contact has a component that is inward or outward.
Also, note that the picture is a little misleading because it doesn't show the low to high component.
Agreed, but first of all I said, “Let’s assume a player rotates straight arm around shoulder joint with locked bend back wrist in horizontal plane and without arm pronation, see please picture below.”
So, it is very simplified version of reality, but I use that to demonstrate the main idea.