It picks a random card, then if the deck cannot generate the required quanta, adds relevant pillars. It adds varying amounts of the card depending on whether the deck is rainbow, the card is a shield/weapon/nymph, etc.). It also often provides quanta generation for abilities.
Later I'm thinking I could make it smarter with quanta balancing (1 pillar -> 5 quanta), teach it how to use pendulums, allow for upgraded decks, let you set conditions (mono Darkness, rainbow, must use this card, etc.).