This core exercise, courtesy of the American Council on Exercise, is called a Side Bridge with Star Crunch.
Begin in a side plank position on your forearm. Your legs can both be extended or you can take a modified version with the bottom leg bent and knee down. Lift your top leg as you extend the top arm overhead. Begin to laterally flex your spine as you pull your top elbow towards your top knee, rotating your knee upward as you bend sideways. Once they touch, begin slowly extending your arm and leg back to the starting position. Aim for 10 – 15 repetitions per side.
Good luck with this one! It targets all of the abdominal muscles by flexing, rotating, stabilising and laterally flexing the spine.