This stretch is done whilst sitting on the ground allowing for optimal stretching of your gluteus maximus. Other minor muscles such as the iliotibial band, semimembranosus, semitendinosus and the biceps femoris, located in your thigh.
Don't ever under estimate the importance of stretching and the benefits it holds.
The Sitting knee-to-chest buttock stretch is useful when dealing with sports injuries such as lower back and muscle strain, lower back ligament sprain, hamstring strain and iliotibial band syndrome.
Always remember that while stretching may be recommended to aid recovery out of injury, you should always be very careful when stretching through rehabilitation. Your soft tissues are likely to be more vulnerable to re-injury if you stretch too much or too hard during this time. Always check with your Health Care Professional who is guiding you through your recovery to make sure it is appropriate to start stretching, and please - ALWAYS FOLLOW THE RULES FOR SAFE STRETCHING (The first posting on this stretching blog).
Image taken from "Lower Body Stretches" wall chart by B. Walker.