I used to be on the competitive synchronized swimming team in high school, and sometimes we had to hold a V position like this and propel ourselves across the pool... it was so hard and I don't think I ever made it more than a few feet! I like this on-land version of a core workout much more! It is inspired by the yoga "boat" pose, but there's no water required, and if you want to take a break, you can just lay down without inhaling water!
Balance in a V shape as shown, with your legs and back as straight as possible. Keep your shoulders rolled down and back. Slowly lower both your upper body and legs to hover above the ground, stopping at the point where your lower back starts to arch. Repeat at least 8 times.