Why does it state that the "Digicue" can be used in competition? Surely not. Same reason you couldn't attach a gyroscope to a putter.
The DigiCue can be used in competition in the sense that since it attaches to the butt and weighs no more than 0.8 oz, it is physically out of your way / line of sight. You might forget it is even there if you are playing well. But as soon as you lose focus and start dogging some shots it'll vibrate if the reason you dogged those shots was due to a stroke problem. The idea is that it is discrete enough that it is effective even during matches.