Stand below ab straps hanging from high bar. Place upper arms in straps and grasp straps above.
Raise legs by flexing hips and knees until hips are fully flexed. Continue to raise knees toward shoulders by flexing waist. Return until waist, hips, and knees are extended downward. Repeat.
Exercise can be performed without ab straps just by hanging from bar. Rectus Abdominis and Obliques only dynamically contract if actual waist flexion occurs. With no waist flexion, Rectus Abdominis and External Oblique will only isometrically contract to stabilize pelvis and waist during hip flexion. It may be necessary to completely flex hips before waist flexion is possible. See Spot Reduction Myth and Lower Ab Myth.
Lie on flat surface or incline to make this exercise less intense.
Exercise can be performed with added weight to increase intensity.