Ideally, in a shoulder stroke with a fixed shape of arms and hands, the arm pits do not open or close in relation to the torso (chest or rib cage). This keeps the putterface square to whatever orientation the chest has, so making square impact is little more than facing the target line with the chest at and thru imapct.
Realistically, people's arm pits open a little and close a little in the stroke. That's okay so long as either a) the arms are not changing relation to the chest right thru impact, or b) the arms are not rotating off line but are swinging down the line thru impact, with the impact zone being merely 1-2 inches befopre the back of the ball to 3-4 inches past that.