Once you have got your boost meter to the max you will be able to make your vehicle go and faster speeds by pressing the LB button. Tapping the LB button once will take away half of your meter, while holding it down will take away more but will make so go faster. In certain situations it's good to tap the LB button (when there is lots of sharp small corners, while in others it's wise to hold it down (like when on a straight road).