Tap on blue cubes, to secure them for the Knobixer. Beware, however on the red cubes for each destroyed Cube removed you from your goal destroyed 3 red cubes do not accept the Knobixer.
Unless you act on the PowerUpSystem with the red cubes. Are than + / - Life highlighted.
You want to complete the level perfectly and completely satisfied with Knobixer. Then have at the end of each level, no blue and green cubes more on the field, and still own at least 3 life.
For more information look in the game under Main Menu / Help after.