The hanging knee raise is a great exercise to work all the way through our abdominals, to help build a strong core, while also working through the back.
Set up for the exercise by grabbing the bar with a strong grip with palms facing forwards. To begin with it is best if you can just about reach the floor with your toes - if you cannot do this I suggest adding a step so that you can. From here, whilst trying to minimise rocking through your body you should pull your knees up and towards your chest. You can then bring them slowly back down and tap the floor to avoid swinging.
As you advance with this movement, you can eliminate the floor tap between knee raises as long as you do not begin to swing. Additionally you can work on bring your knees up higher and even closer into your chest.