That is - in fact - the rule. There is a requirement that it be deliberate. However, when determining intent, making yourself "big" by havin your hands away from your body to make it easier for the ball to hit the hand counts too.
There is a lot of talk about hands being in an "unnatural position", but that doesn't really matter at all. What matters is if a player's hands are away from his/her body in a way that allows the player to more easily block a ball traveling nearby. It is expected that players will pull their hands into their body to avoid contact, even if that position isn't natural.