How does Drawing Duplication (Mentalism) work?

I recently saw a mentalist who told a spectator to draw any symbol she wants on a piece of paper. The mentalist guessed it was a heart, and indeed it was. Now I've seen these tricks before and I think I know how they work if a clipboard is used or if the mentalist wears a fake blind fold, but this time there was no clipboard, just a normal piece of paper and the mentalist was facing the audience all the time. I also don't think he had a confidant in the audience (we're all students and know each other). So how does it work? What made me suspicious was that I personally would have chosen a heart too.