The scissor kick is often used to target the lower abs. However, this exercise does a poor job of working the abs. The hip flexors do most of the work. This exercise can be used in a continuum of ab exercises, but it should not be used as a primary ab exercise.
Begin by lying on an exercise mat. Place your hands under your buttocks. This will help with leverage by reducing the load on the lumbar spine. Raise both legs approximately 15 to 20 inches off the ground. Contract your abs, exhale, then kick both legs outward as if you were opening a pair of scissors. Bring your legs back to the center and cross them over each other (closing scissors). Perform the scissor kick in alternating fashion. Be sure to keep your abs flexed at all times.
Lie down on a mat. Contract the abs. Exhale. Kick legs outward.
Cross legs in the center. Repeat the exercise.