How do you Unlock the Bonus Levels?

You will enter a Bonus Level automatically when you have collected a Bonus item and then completed the level in which it was found. When you enter a Bonus level your character will fly forward non-stop through an obstacle-free space that has no enemies for 15-25 seconds where you will come across various clusters of Rings and Super Rings in certain formations which you will be able to pick up to rank your score or unlock content.

In the Bonus level you move your character up by tapping and holding the screen. To move your character down you release the screen.