Lie supine on bench or mat. Place hands under lower buttock on each side to support pelvis.
Keeping knees straight, raise legs by flexing hips until hips are completely flexed. Return until hips and knees are extended. 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 leg-hip raise. Attempting to perform Lying Straight Leg Hip Raise would not load waist and hip flexors after they travel beyond vertical so that sort of movement would need to be performed on steep incline or vertical position, requiring much greater strength. Also see Spot Reduction Myth and Lower Ab Myth.
To decrease intensity, flex knees along with hips. Alternatively, perform exercise on floor and allow heels to make contact with floor each repetition.
Unless ankle weights are used, weighted version of this movement is not recommended due to potentially vulnerable position if dummbell accidentally slips from feet. To increase resistance, perform on incline board. Knees can also be raised to shoulders to dynamically involve Rectus Abdominis.