Mary Lou Williams was a groundbreaking composer, arranger, performer and mentor. She was also known as "the lady who swings the band," thanks to her mastery of swing, "jazz's unique approach to rhythm," says Helen Sung. Sung is a renowned jazz pianist and award-winning composer who won the Mary Lou Williams Jazz Piano Competition in 2007. We asked her to explain what "swing" is and what makes it so central the sound of jazz, and to show us how to do it.