How do you get top secret in bloons tower defense 3

First you buy the regular monkey beacon, then you upgrade it fully. Once you have done that the question mark at the bottom of the screen will turn into a B with lightning coming out. It works best when your screen is full of balloons and you are afraid that you will lose lives. You can simply just put it on the screen and nine monkeys will flt and wipe out everything but the blimps with brown balloons.