You should feel the stretch on your semimembranosus, semitendinosus and biceps femoris in the back of your leg all the way from you knee to your hips. You should also feel minor stretches through your intertransversarii, multifidus, rotators and obliques of your back and abdominal.
Don't ever under estimate the importance of stretching and the benefits it holds.
The Sitting Side Reach Stretch deal with back muscle strain and back ligament strain.
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.