Start position is lying on the back with the hips raised off the floor and body supported by the upper back and feet, keep the head off the floor with the chin tucked in.
Lift one leg and straighten at the knee.
Keep the leg aligned with the trunk.
Do not let the pelvis sag down to the side.
Hold this position, then switch legs without lowering the hips, as long as your form remains perfect, continue on switching over between legs.
Advanced techniques include placing the arms overhead and the heels further from the body when commencing the exercise.