Central Line Theory
In all of the forms the hands are crossed down and up to define the "Central Line". This IS NOT a block.
When the fingers are even as in the yellow lines, you can strike the same point in space with both fists without telegraphing your intent by moving your shoulders.
Guarding the Center
Guarding the center forces your opponent to take the longer route. Your opponent must strike in an arch, giving you the advantage of striking the opponent's center.
Your guard hands should be extended about 3/4 in the center of your body making your opponent have to take the longer route in order to strike you. Also you will avoid the close-in strikes used in boxing.