From what I've read, the cause for the force acting as the result of the Magnus effect is the formation of a bended wake behind the moving sphere.
Take a look at that picture:
From that picture, it appears to me that since the pressure gradient is directed into the wake region, the vertical force acting on the ball should be directed downwards, and not upwards how it is indicated in the picture.
Is this picture wrong and the wake should be reflected about the x-axis?
From my understanding, the effect responsible for detachment of the boundary layer is the development of turbulence, and turbulence should occur faster on the side where the relative velocity of the air is bigger - on top of a backspinning ball, since in that case the ball will be giving its momentum to the surronding air faster, through friction, therefore reaching the speed where turbulence occurs at the point of the x-axis located more to the right (and not as indicated on the picture, where it's more to the left (on the top side of the ball)).
Edit: Sorry, the last paragraph is wrong. I incorrectly determined where the relative velocity is bigger. My question therefore changes to the following - If this picture of the wake is correct, then why is the force directed upwards, and not downwards?