This stretch is a nice one to counteract a long day at your desk/on the computer. Indeed, we often reach forward on a day to day basis, and the shoulders tend to roll forward. This stretch is the exact opposite movement, and is a nice reminder to the shoulder that it can still move backwards and open the chest.
This stretch can be adapted to act on different fibres of the pectoral muscles, by bringing the elbow slightly higher on the door frame than demonstrated.
Be careful when you do this stretch not to feel pain, neither should you feel stiffness at the top/back of the shoulder -- the stretch should always be felt at the front of the shoulder and into the chest area.
Do this stretch at your own risk. A stretch should always be comfortable. The idea is to tame the muscles into a sense of relaxation and not to force it. Always do both sides when you do it. I recommend holding it for 10 to 15 seconds only.