It's called that because when you press a key, rock bars cause the plates on the assembly to move so that the same number of teeth as the key you pressed will be selected to engage other gears.

Note well the orientation of the parts relative to each other!

Start bag 20 and 20A. 20 will be for the parts, while 20A will be for the large separating springs that you'll see shortly.

Remove this taper pin.

This will let you slide off two parts: the crank, and the bushing. Bag them with the taper pin.