This stretch is done whilst sitting or standing anywhere and stretches the pectoralis minor and major mainly. You should feel the pull from the center of your chest towards you shoulder evenly. It also stretches your latissimus dorsi and serratus anterior, which are located down the side of your torso and then anterior bicep on top of your shoulder. All these muscle are involved and work together and it is very hard to isolate just on muscle or use just one muscle without using a few of them.
Don't ever under estimate the importance of stretching and the benefits it holds.
The above head chest stretch is useful when dealing with sports injuries such as impigement syndrome, rotator cuff tendonitis, shoulder bursitis, frozen shoulder, chest strain, pectoral muscle insertion inflammation.
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 "Upper Body Stretches" wall chart by B. Walker.