This is a great drill for increasing power on the serve.  The purpose of the drill is to hit the back fence on the full. Straight away you’ll notice an increase in racquet head speed, it may be tougher to hit the back fence than you think but keep at it and remember to keep your arm and wrist nice and loose, don’t force it. Once you’ve hit the back fence go back to trying to get the ball in the service box, you’ll notice an increase in power.