Ab blast, a video showing Hanging Knees to chest abdominal exercise.
Just one video of many that Fit for You have put together to teach you the correct way to train ab’s and get a toned stomach

Using a chin bar, hang with arms slightly flexed, breathe out as you contract stomach muscles bringing knees to elbows.

You can use a slight pushnoff the floor if you are just beginning, and once you’ve mastered the exercise without pushing off the floor, to make the exercise more challenging, you can simply slow the pace of negative action (lowering the legs) or harder still, slow the positive movement.