Ok see, this is the problem.
It shouldn't be a new concept to you that you are pulling the shoulder blades together (retracting them) on a row. Both bench and rows both involve scapula retraction.
If I had my way I'd rather you not worry about deweighting it at all and learn how to do a row period before you do JS rows anymore. Then if you want to do the whole upper back rounding thing, do it at your own risk. You could deweight by putting the bar on something raised and then do the row with out all the thoracic rounding.
Some thoracic rouding at some points, like sometimes when your doing a particular heavy deadlift, is to be expected. But to do it as PART of an exercise is really unnatural. Your back wasn't made to continually flex in this fashion. I knew better when I started to do them in the first place...but you live and learn.
The big problem is the distinction between JS rows and parllel deloaded rows. It is perfectly feasible for many, if they have the flexibility and strength to perform deloaded parallel barbell rows without all that thoracic flexion. I do it all the time. So many people keep saying that the Pendley/JS is just a deloaded row. Well if that is the case then fine.
Unless these people think that a barbell row is always performed with a thoracis flexion/extension. Then I have to wonder what the hell is going on here. From what I have been able to get, the distinction between JS rows and regular parallel rows is the upper back movement. If you think that the back was meant to function like an elbow joint then by all means keep on keeping on
.