It's force of will combined with self-confidence. Sorcerers cast magic using "The Secret," basically. They believe in themselves hard enough that they manifest power into being. That's the idea of charisma spell-casting. It's okay if you think that's dumb, but that's the idea at play here.

edit for an analogy: where a Wizard has a magic book full of spells, the Sorcerer has a glitter-dusted vision board.

second edit: added links to give context for anyone unfamiliar with relevant terminology