2.6.5 As soon as the ball has been projected, the server's free arm shall be removed from the space between the ball and the net. Note: The space between the ball and the net is defined by the ball, the net and its indefinite upward extension.
The accompanying diagram shows two separate serving locations, and how the space between the ball and the net changes depending on the location of the ball.
In essence, this rule has made it illegal for the server to hide the ball at any point during the service motion. Provided the receiver is standing in a conventional location, he should be able to see the ball throughout the service action.
Note that the rule says that the free arm shall be kept out of the space between the ball and the net as soon as the ball is thrown up. This means that you must move your free arm out of the way as soon as the ball leaves your palm. Unfortunately, this also appears to be one of the most commonly violated rules by players, and since the umpire is side on to the server, it is not always easy for the umpire to be sure whether a player is getting his free arm out of the way. But, as mentioned before, if the umpire is unsure whether the serve is legal, he should warn the player, and fault the player for any future serves of doubtful legality. So get used to getting your free arm out of the way immediately.