Grasp and hang from high bar with slightly wider than shoulder width overhand grip.
With knees straight, raise legs by flexing hips until hips are completely flexed or knees are well above hips. Return until hips are extended downward. Repeat.
Rectus Abdominis and Obliques only contract dynamically if actual waist flexion occurs. With no waist flexion, Rectus Abdominis and External Oblique will only act to stabilize pelvis and waist during hip flexion. It may be necessary to completely flex hips before waist flexion is possible as in hip leg raise. Also see Spot Reduction Myth and Lower Ab Myth.
Knees can flex along with hips to decrease intensity. Alternatively, exercise can be made easier by lying on flat surface or incline board.
Exercise can be performed with added weight. Knees may also be kept straighter throughout leg raise to increase intensity. Knees can also be raised to shoulders to dynamically involve Rectus Abdominis.