The guy in our guild presentation said it had to do with the way the thread plys are twisted together---they are designed to glide smoothly in the direction from spool to needle; but when pulled 'backward' up through the tension discs that small bits of the thread gets shredded off and will eventually cause problems. News to me!
If this is true, I guess we should never sew by using a wound bobbin as a thread source thru the needle- guilty!!!!!
