I periodically see references in threads about rear derailleur adjustments to adjusting the B Screw. I understand basically what it is supposed to do with regard to relative position of pulleys to casette. However, not once in my early attempts at things mechanical did an attempt at B Screw adjustment have any effect on derailleur function. Then I spent some time at UBI in a bike mechanic class. The guy who taught the section on derailleurs commented that he had never found tinkering with B Screw to have a meaningful effect on derailleur function, validating my view that it is basically pointless. So, what am I missing. Under what circumstances does the B screw actually effect derailleur function in a meaningful way?