Yoga term: Matsyendrasana
This is a good stretch for the side of the leg to the area above the hip and sacrum. I can be a bit tight here at times so I like to start with a lighter variation where I hug the leg that is up. Then work to placing the elbow behind the knee to go further into the twist. The most advanced variation of this pose ends with a lock that is made between the opening through the opening created by the legs.
The butt should be fully seated on the floor. Back is straight and vertical. Keep chest out. Head is straight and fully turned in the direction of the twist with the chin slightly tucked. The hand that is not on the knee is placed straight down and behind the center of the body.