The advantage comes from knowing in advance how the throw will be made — whether short and fast to the front of the line or looping slowly to the back.
For example, the code-word "postman" might indicate that the ball is to be caught by the jumper (typically number four) and held briefly while a player from the back of the line-out runs along the line.
There are also numerical codes, and more esoteric ones involving anything from the number of legs on animals to the personal characteristics of people known to the team.
These basic elements will be combined by the caller into a phrase intended to confuse the opposition while transferring the plan to his team.
Most of this phrase will be "padding" intended to hide the true method - for example, "127 ZD Kangaroo" could be decoded simply as a throw to the front of the line.