I'm a former competitive MtG player. My playgroup found that with a bit of practice and a specific technique, we could roll 17+ on any spindown dice almost every time (try rolling the die with it starting in your hand with the 20 facing upwards with a sideways motion). While technically you can use any method you choose to randomly determine the start of a game, these dice have been proven to not be "random" thus cannot be used in a tournament. It doesn't matter at all in casual play, but in a tournament, you're just giving your opponent a chance to play first for free.