Kakarat - it's obvious your intentions are good and you put some thought into your comments here.
However, there are some things I respectfully question.
Why do you think the OP needs to stretch the posterior capsule as explained in the DeFranco video? He explains in the vid that it helped him quite a bit with his shoulders.
However, NOT everyone who suffers from shoulder problems also suffer from tight shoulder capsule issues. More mobility in this area can actually be a waste of time or create more harm depending on the individual.
I do not believe this movement has a sufficient risk/reward ratio on an individual (the OP) who has issues that are not adequately diagnosed.
I do take exception to any trainer that gives this movement as a matter of course. More mobility in the posterior capsule is not something that everyone needs.
Did Lee Boyce actually name that raise after himself...? If he did, he may not realize that others have been prescribing similar movements long before.
This is actually a variation on the Y movement to engage the lower trapezius.
I could be wrong on the source; however, the first time I saw this variation was on a Poliquin article. In that variation, the forehead is actually resting on the arm that is on the bench. This is a BETTER variation because, by resting the head on the arm, the upper traps are not engaged and thereby do NOT interfere with the action of the lower traps (upper traps elevate; lower traps depress). And because the upper traps are so much larger it is imperative that the person doing this movement does as much as possible to take the upper traps out of the equation.
Another reason I'm not crazy about Lee's version is the scap retraction prior to the abduction of the arm. There are better methods to engage/strengthen the scap retractors. And, if the purpose of doing this movement is to better engage the lower traps, the trainee is better off: a) resting the head to better disengage the upper traps; b) have someone possibly tap his lower traps as he does the movement to better build a feel for the muscles engaging.
A common method I employ is to combine autogenic inhibition prior to the movement.
Like I said, if Lee actually did name this movement after himself (I don't read his articles), he's only embarrassing himself.