The Bonus Multiplier boost puts the first multiplier gem on the board at the beginning of the game (the 2x multiplier gem). As with any other multiplier gems, you'll still need to destroy it to get your multiplier up and start receiving the bonus points for gems you destroy. And as with any of the other boosts, this will last for 3 games (and another 3 each time you buy or auto-renew the boost).
Points are only multiplied for gems destroyed after the multiplier gem has been destroyed (and not the score over the entire game), so getting the multipliers as early as possible will help your overall score.
Also, it means you'll need to trigger 1 less multiplier gem to reach the max (8x). Multiplier gems normally drop when you destroy 12-18 gems in one move (depending on how many multiplier gems you have had out already).
