Just a thought here...
Whatever happens on the left side of the body, is a counterpoint to what happens on the right....
So for those of you who are having difficulty understanding some of the stuff above, take a view from the other side...
A "right fielder" tends to compress the right side too early on the downswing (i.e. bring the right shoulder under more)....
Compressing the right side WHILE maintaining the swing center position just forward of your 7th cevical bone, has the natural side effect of stretching the left side (think of your shoulders as being spokes on a wheel, with your neck being the central axle, so when one spoke goes down, the other comes up...and when the shoulder comes up rapidly, so the left side stretches....
Another way of "not stretching" the left side too early is simply to stop compressing the right side too early.
My own "cure" for this was simply to modify the right shoulder track from the top, i.e. more outwards than downwards...This stifles the right side compression (or left side stretching),,,it also stops all the fats you get with early right side/shoulder under....