Players received a limited number of them at the start of trading, similar to the distribution of Trade Tokens
That's Orim's Chant in a nutshell: good for stopping an opponent in the middle of his combo while he has a bazillion mana floating and damned if he isn't going to take mana burn and die, but don't you just wish it was a cantrip
Let's create a very simplified situation as an example of what I mean
What were they doing before they found their way into our Magic: The Gathering games
However, when a player chooses to Mega Evolve a Pokmon-ex, the players turn ends immediately unless the Spirit Link card that corresponds to the Pokmon to be Mega-Evolved has been attached to the Pokmon in question
Belbe, Corrupted Observer Belbe, Corrupted Observer gives out free colorless mana for people dealing damage, up to 6 for you and 4 for your opponents