One way to adjust the tendency to pull the disc forward, raising the leading shoulder and tensing the muscles - is to take your off arm hand (your left hand) and grab your right arm inside the elbow, as if you were crossing your arms.
Keep your off arm adjusted so that your throwing arm is locked at 90 degrees from your shoulders.
Now you have a feeling for the arm angle in the back swing, and how it should stay shaped as the disc comes forward and through the release. Most players will at some point or another have issues with that 90 degree angle between their humerus and their clavicle collapsing to some degree.
When I think of people asking, "what muscles do I engage" with the upper body, it's hard to explain that your firming up the frame of the 90 degree angle while still keeping the forearm loose and free - so we will often describe that as keeping the disc out wide in the backswing.
The result of the wide backswing is that we are getting our 90 degree angle set. If you collapse the 90 degree angle to "pull" the disc forward - that front side shoulder will raise or tighten.
This thread is so freakin awesome. I've learned so much just reading this past week. HUB, by what I bolded above - So during wide reachback then the positioning of elbow is then set and will not change, while the forearm will be loose and going from out to in to out - correct? (And I understand result of all of this is setting of 90 degree angle, I'm just clarifying body parts positioning)
And is attempting to collapse 90 degree angle during pull bad (I don't think I could get much tighter angle anyway)? Slow hope you don't mind me clarifying on your thread.