Position ball against sturdy support. Lie with back on ball and heels on floor with legs straight. Grasp sturdy bar(s) near head for support.
Raise legs by flexing hips and knees until hips are fully flexed. Continue to raise knees toward shoulders by flexing waist, raising hips from board. Return until waist, hips and knees are extended. Repeat.
When raising hips, keep knees fully flexed, so as not to throw weight of lower legs over head. Rectus Abdominis and Obliques dynamically contract only 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. Also Incline Leg-Hip Raise on incline board.
Exercise can be made easier by positioning body so it is orientated more horizontally. Alternatively, movement can be performed on bench.
For greater difficulty, body can be orientated so torso is at greater elevation. Exercise can be performed with additional weight or movement can be performed in vertical orientation.