The magnus effect – a curved ball explained
In the diagram of a spinning ball or tube (see video for both examples) the stream of air rushing past tends to cling to the spinning ball or tube and so it deflects the stream of air downwards. The force pushing the air down results in an equal and opposite force pushing the tube up. This gives rise to the Magnus force.
At the top the rotation is away from the direction of flight so the speed of the air relative to the surface is low and so the air tends to stick to the surface.
Because the bottom edge, as it appears in the diagram, is spinning towards the motion through the air, that edge is moving faster relative to the air so the stream breaks away. This produces turbulence behind the ball and the air is not deflected very much. The overall force on the ball, therefore, is pushing up in this diagram.
There is a more thorough explanation with practical examples in the video below: