Reversible Gear is the 12th level of Advanced in Super Monkey Ball 2 (41st in Super Monkey Ball Deluxe), 2nd stage of Under the Ocean in Story Mode (19th in Super Monkey Ball Deluxe), and 101st stage of Ultimate without continues (81st with continues).
This stage features an octagonal platform. There is a big starfish-like gear that spins around, stops, and reverses directions. There is a gap allowing for the monkey to roll into the middle where the goal is located, and there is a single banana on the way to the goal between the gear's teeth. There are banana bunches between the gear's teeth in the rest of the locations.
To get to the goal, roll onto the platform with the spinning gear. Turn a bit to the left or a bit to the right, but a bit farther along the path if you go to the right. Go to a part of the platform where the gear's opening is lined up exactly with you when it is stopped. From there, roll at full speed once the gear's opening is about a second before it stops and into the goal.