To be able to do this you have to press L1 + X and if timed correctly you will go into a crouching position and spring higher than you would by double jumping (X, X). If you do the rolling jump and press X just as you touch the ground you will jump as high as the double jump. Press Circle rapidly after double jumping and you will do a longer jump.

Avoid Fall Damage:
Just before you hit the ground press Circle to spin kick and you will land perfectly.

Shoot Cannons Farther:
Hold Square or Circle + X to make the cannons at Sentinel Beach and Misty Island shoot further. This is helpful with any steel boxes.

Alternate Ending:
After collecting 101 Power Cells successfully complete the game to view a different ending.