First, you need to know what the blast meter is. The blast meter appears in multiplayer maps and in Marble Blast Ultra levels. As time passes, the meter slowly fills up, and when it lights up, you can press E to blast yourself in the air slightly. More juice in the meter means a bigger blast. You can also push back other players with this if they're close enough.
The Blast powerup, also known as the Ultra Blast, will instantly overfill your blast meter, allowing you to go even higher and push back players with extra power. Most of the time you will find these scattered throughout many multiplayer Gem Hunt maps.
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