How to work on abs and core with the half jackknife

Take a look at this instructinoal video and learn how to work out your abs and core with the half jackknife. To perform this exercise, sit on the ground with your knees bent and feet on the ground. Your hands should be on the ground at your sides or slightly behind. Then raise your feet off the ground and bring your knees toward your chest. Extend your legs out straight and parallel to the ground, and repeat. You can make it harder by raising your arms above your head.