Hiding the ball illegal

Hi, I have seen some players who raise their non-playing hand when serving harder, isn't it illegal? beacause you are not allowed to hide the ball behind your arm.

AN EXAMPLE: https://www.youtube.com/watch?v=WmHMV9OYRz4&t=305s

Hi Anton,

It is hard to tell from the angle of the footage but you are not allowed to hide the contact of the ball with your free hand.

It is a rule that is difficult for the umpires to judge from where they are sitting as well.