Hang from a chin-up bar using an underhand grip (palms facing you) that is slightly wider than shoulder width.
Now bend your knees at a 90 degree angle so that the calves are parallel to the floor while the thighs remain perpendicular to it. This will be your starting position.
As you exhale, pull yourself up while crunching your knees up at the same time until your knees are at chest level. You will stop going up as soon as your nose is at the same level as the bar. Tip: When you get to this point you should also be finishing the crunch at the same time.
Slowly start to inhale as you return to the starting position.
Repeat for the recommended amount of repetitions.
Variations: You can also add resistance by holding a dumbbell or medicine ball between your feet but this should only be done by the most advanced athletes. Alternatively, when you are very advanced you can also try the one-arm version where you grasp your wrist with your free hand and you work the obliques more.