Mirror Mode is the same as regular mode, except that you have two "mirror" buttons. One causes the path (along with any bloons on it) to flip over to its mirror image (leaving your towers and special agents where they are, even if that means they would be on the path or the wrong terrain), and the other causes the overall positioning of your towers and special agents to flip over to its mirror image (leaving the path where it is, and again allowing towers and special agents to be on the path if necessary). Effectively, this allows you to reposition them with respect to the path. The buttons can be used whenever you want, but you must wait 10 seconds for the buttons to "recharge" after each time you use one.