/diy/ - Do It Yourself

Let's say I had a setup like in the picture, where Gear A is driven by the motor, and gear B is concentric to gear A. How do I make it so that I can "couple" A and B together at will?
I want B to be free spinning most of the time and be driven by A on command, maybe a solenoid or something that couples the two together somehow?
any pointers would be appreciated