Well I must admit that our terminology is apparently not in agreement. I always thought that the parallelogram was the parts of the der that form, well, a parallelogram - two arms, fore and aft "body" segments, and four pivots making them into a moveable rhombus.

OTOH, I think he is saying the same thing I did about the movement of the idler and chain farther back and down from the sprockets using the screw in question. That seems to provide a tad more flexibility in the chain line from idler to sprocket, which may allow for better shifting across the range.

It is certainly true that making that adjustment eliminated the inability to shift smoothly for more than a few adjacent sprockets. Well, about half at a time, anyway.

[Edit: In my continuing quest to fend off age related stupidity in the use of the English language, I have to point out that to be a rhombus, you have to start with a rectangle all of whose sides are equal. A square. The bike part in question is simply - dare I say it - a generic parallelogram. Sigh. ]