Take your 2009-2012 Racing Rules of Sailing rule book and look inside the back cover (fold out). These are the Race Signals, which are just as much rules as any other part of the rule book.

You'll find a section like this:

I have a plastic board with the green triangle painted on one side and the red square on the other. The gate boat will hold it up to display the appropriate symbol for the direction of the change mark. It's so much faster and easier than having them write a compass heading on a white board.

As a competitor, I've gotten to the point where I hardly look at the boat signalling the change. If I hear the horns, I start looking for the change mark. I certainly don't have the time to read a compass heading, remember the compass heading at the start and mentally figure out which way the mark's been moved. If I see green - mark's moved right; red mark's moved left. That's easy.

This stuff is not new. It's been in the last couple of rule books and is even on the HCA Flag Signals stickers (although I think the two different shapes didn't get introduced until 2004 - the different shapes are for people who're color blind.)