I do this a lot when I'm watching TV. I lie on my back sometimes with my hands behind my head or to my side. I put both of my feet straight up together in the air (perpendicular to my body, I look like an L). Then I separate both legs to go to either side of my body as far as I can sometimes and I will slowly bring them back up to meet together.
I've been doing this for while, and I want to know the term for it to make sure others are doing it and that I'm doing it correctly.