You'll need to unlock the ability to use fireball power-up first by finishing matches and gaining xp. Once you've done so, allow yourself to get the fireball power-up in a match. After you've got it you can fall back and explode to reset positions. After the new round starts you'll have to wait a bit to use the power-up so stay back just a little. Also, make sure that the path does not immediately go up a wall as this will destroy the fireball. Use it soon enough on a straight enough part of the track and you should be golden!