We called these "leg lifts" and would do them hanging from the high bar.
- Start by hanging on a pull-up bar with palms facing out, or use the ab straps and hang with your arms inside of them. Do use a box or bench if you need help reaching the bar.
- Pull your shoulder blades down your back to keep your shoulders away from your ears.
- Engage your abs as you fully extend both legs, lifting them up simultaneously with control.
- Lower the legs down slowly and return to the starting position. This completes one rep.
- Avoid swinging during this move to ensure that you're really working your abs.