"Now how it works is, we're going to take turns dancing as fast as we can. But we won't be dancing to fast songs. We'll be dancing to slow songs from the '70s," Fallon told his guest, who danced off against the NBC star over a year ago. "They've all been picked by Questlove, who has chosen the least danceable songs that he can think of, and the best fastest songs he can think of."
After some confusion from Lopez, who was promoting "World of Dance" and her new single "El Anillo," Fallon clarified the object was to simply dance quickly to slow jams. Lopez decided she would one-up Fallon's speed because she was in it to win it.
"OK, Jennifer, you will be dancing to the classic "Three Times a Lady" by Tuskegee, Alabama's own, The Commodores," Questlove told Lopez, and she proceeded to deliver sped-up booty swaying to the Motown classic.
Shockingly, Fallon may have upstaged Lopez. As Questlove played the next song -- "Babe" by the Styx -- Fallon shuffled, boogied and spun like his life depended on it. Fallon later admitted to Lopez that he's "so competitive."
Watch the video above to see who was declared the dance-off winner, and to see a final synchronized dance duet to Peaches & Herb's "Reunited."