Grab a pull-up bar with a double underhand grip at shoulder width. Hang from a bar with your legs extended.
Your arms should be also relatively straight.
Initiate the motion by retracting your shoulder blades, and driving your hips up and forward. In other words, using your lower abdominal and core muscles, tilt your pelvis under, bend your knees and raise them (while crunching your core) as as high as you can in a smooth, controlled movement.
Bring your knees up to the tips of your elbows.
Your hips will naturally move forward slightly, but don’t let the momentum swing your body.
Hold for a second, then slowly return to a full hang/full extension through the elbow and shoulder before repeating.