Imagine a "tug of war", one team pulling against the other side. Often times with upper back tension, the "tug of war" is between upper chest muscles and upper back muscles. What frequently happens is tension in the upper chest muscles are a component that can pull the shoulders forward and contribute to a tension in the upper back/shoulders/lower neck region. (See The Nose Ring)
Massaging this point in a downward direction encourages the chest side in this "tug o' war" to ease off, allowing the other side to also ease off. I suggest using the hand/fingers on the opposing side...for left subclavious, use your right hand.
Note your tenderness, try the massage again in a few minutes and the tenderness usuallydecreases. Since tenderness is often a signal of muscular tension, realize that within just a FEW minutes, you have REDUCED your muscular tension.
...with a bit of practical information, pain and tension can simply ease away.
Please understand this self help technique is best regarded as a 'band-aid', that upper back tension is often an indication that your primary balance is off .
Get some bodywork!!!
Body/muscular tension monitoring
About muscles
A shopping list of muscles to offer to your therapist