Hang from a bar with an active hang. Keeping your legs straight, lift them until they're parallel to the floor (90 degrees) or higher. Lower them slowly with control. Don't swing—use your core, not momentum. This builds serious lower ab and hip flexor strength. If you can't keep your legs straight, regress to knee raises. Work up to 10 controlled reps. This is one of the best core builders in calisthenics.