Use the abdominal muscles purposefully when exercising. The muscle fibers are long so you want to fully extend and contract with each repetition. Using the disc in the first exercise on the right allows for longer rectus abdominus extension. Be mindful that you do not release your hips when you lean back. The motion of simultaneously extendiong one leg while extending your back stabilizes the pelvis AND works the hip flexor ( Iliopsoas) muscle. Flex the leg at the hip while you contract the abdominal muscles and raise the tosro to complete the exercise. Be sure to keep the neck relaxed. Do a complete set of 10-15 repetitions with one leg before switching to the other side.
The double crunch with two balls is another variation working the abdominal and hip flexor muscles but also adds the inner thigh muscles (adductor muscles), and even the back (Latissimus Dorsi). You start extended on a mat holding a ball in you hands, with a second between your ankles. Both balls can be weighted if you prefer. As you exhale you contract the abdominal muscles and raise up keeping the arms and legs straight ( forming a "V" with your body ( aka a V-up). Chris' "V" looks like a lopsided "U" because he is using two props that vary in size and weight. This is appropriate.
Standing with feet shoulders width apart, and knees slightly bent, keeping the arms straight in front of you just below the height of your shoulders, and keeping your hands next to each other, twist your torso away from your anchor then return to center. Try not to move the lower body at all. That Bonnie doesn't move her head is a bonus, feel free to move yours. This move can be done just as easily on a cable apparatus. You are strengthening the Transversalis or abdominal obliques.
