Extract, protein, and head retention.

[mixedbrew]

I was listening to the brew strong pod casts and I could have sworn I heard something but now I can't find it. It was John Palmer talking about late addition extract, when and why you add it to the end of the boil. He breifly mentioned protien and head retention. I am pretty sure he said something about not wanting to cook the middle sized proteins out so you will have a better head. Am I losing my mind? Or is there something about this? Thanks