From a sit-up position with your legs slightly bent, pull one knee to your chest and grab it with both hands. Flex your toes, keeping the sole of your foot pointed forward. Release your knee and bring your foot all the way down, planting it as you lean back.
Advanced version: After you pull your knee to your chest, push it into a kick, leading with your heel before you plant your foot and switch legs.
Brett's Tip: Keep your chin tucked with your eyes forward. That keeps your shoulder blades off of the ground and your abs engaged.