Ahhh! Ahhhhh! I am extremely reluctant to go into long explanations here. Have you read the TDT manual though? I think it's up at the singinst.org website now, finally. It might dissolve confusions of interpretation, but no promises. Sorry, it's just a really tricky and confusing topic with lots of different intuitions to take into account and I really couldn't do it justice in a few paragraphs here. :(