This achievement is obtained by bouncing the fin back-and-forth between ibb and obb, without touching the ground.

1. Play as single player, and choose local co-op.
2. Go to Secret Crystal 2, or any level you prefer as long as there is enough space.
3. Bring a fin to the top platform. Any fin size will do, but personally I prefer the big one because it easier to see.
4. Start bouncing by simultaneously move the ibb/obb to the left and right together. Note that there is no need to jump.
5. Keep doing it until the fin bounced 10 times, or until the achievement box appear.



By practicing several times, you will get the correct timing when to move the characters.
HTH :)