The Mystery Gift System works by an online prize distribution from the makers/creaters of Pokemon.
There are several types of mystery gift transfers:
- via. local wireless
- via. internet
- via. code
- via. infrared
Using these options, you decide how you want to receive to gift. The most common one that people use is via. internet, since its free to everyone and only available for a certain time. You will get notifications from Pokemon sites telling you what has been released and when it will end.
Via. code is used for codes that the makers have given out, or finding a code when you watch certain movies/films etc.
You can also send gifts to friends when you received a card, therefore they can enjoy is too.
Basically, the mystery is a free gift system where people are free to earn rare Pokemon and gifts.