I understand the concept of leaving your clippings on the lawn... makes sense;

I don't understand when and where thatching (or de-thatching?) should be done. I seems to contradict the concept of leaving clippings n the lawn?

If you over fertilize and over water Kentucky bluegrass, it can form a dense undergrowth called thatch. Other grasses, or even KBG that is taken proper care of, really don't have a problem.