Its name also describes the difficulty. It is arguably the most difficult level in the game! Have good timing skills, bring Yoshi, have a cape! This is the first (and last?) time you're going to meet walking fire in this game!
In order to pass this level you have to feed Yoshi the second spring that's among the bullet bill cannons. To pass the cannon that is right nex to to the spring board you have to jump on a bullet bill at the right time. Be hasty while clearing the rest of the cannons, otherwise Yoshi will swallow the spring.
Probably the easiest (and least satisfying) way of passing this level is to fly over all the action - just make sure to clear a column of blocks about halfway through