If you hit the plus with your ball, your ball will bounce back under the screen. This is very useful to hit multiple balls because then they are not gone immediately. Use this in combination with the extra balls to clear the field quickly.
You can get multipliers in Bricks n balls, with this multiplier you shoot multiple balls in a row. Once unlocked, keep this multiplier. This is very handy because then you can quickly remove the blocks with, for example, a 10 on it. Otherwise this will be almost impossible