This little video has information about tension - both bobbin and top thread - that I had never seen anywhere else. It is specifially for a long arm, but seems that it would also be true for FMQ on a domestic machine. If that's not the case, please tell me. I learned a LOT here!
Dollyo posted this link on another thread so thanks!
http://www.youtube.com/watch?v=Q1mRhcquZTM