This exercise is magic for neck and shoulder tension, and after doing it once, you will feel that the weight of the world has been lifted from your shoulders.
Although not described in the video, you may use single shoulder contractions before then trying to pull that shoulder down, using the muscles under the arms. The reason this exercise is so effective is we are showing the body how to stretch itself, and we have found that the more you can encourage the body to do this, the longer lasting the effect is.
Find the audio file of this video at https://supplementary-material.s3.us-west-2.amazonaws.com/Shoulder+pushdown+HD.mp3.