[The video was shot outdoors. The background noise is the sound of wind blowing.]
Muscles worked : Primarily abdominals and hip flexors; also elongates and decompresses the spine; strengthens your grip.
Grip a horizontal bar that is high enough so that your feet do not touch the floor.
Knee Raise : With control, bring your knees up toward your chest. Then lower them. Do not let your body swing to gain momentum. Be smooth. Inhale and exhale with control. Do not hold your breath. Work towards 10 to 20 reps.
Leg Raise (more difficult): With almost straight legs, bring your feet up to the bar, and with control lower them. Do not allow your body to swing for momentum. Work up to 10 to 15 reps.
Note: Most gyms have dual arm harnesses hanging from the chinning bar. If your grip is not strong enough to hold your bodyweight for an entire set, slip your arms through the harnesses and do the exercises without having to use a hand grip.