Important stabilizers (not highlighted): Rectus Abdominis, Obliques
Lie on your back (supine) on the floor.
Place your hands under your buttocks, with your palms flat on the floor.
Press your lower back against the floor.
Raise your feet a few inches off the floor.
Breathing naturally and keeping your knees almost fully extended, kick your legs up and down as you would if you were swimming. As your right leg rises, your left leg should descend; then, as your left leg rises, your right leg should descend.
Comments and tips
Keep your lower back pressed against the floor. This helps to protect your lower back.
Placing your hands under your buttocks supports your pelvis.
Do not allow your legs to touch the floor.
Instead of kicking your legs up and down, parallel to each other, you can kick your legs around each other in alternating semi-circular motions (that is, as one leg goes up and in, the other leg goes down and in, and then the movements are reversed).
To make the lying scissor kick more difficult, use ankle weights to increase resistance.
Your rectus abdominis and external obliques will only act as stabilizers since there is no flexion of the waist.
The lying scissor kick is a fantastic exercise to use to burn out your hip flexors.