I subscribe to the Voiceovers group on the Yahoo newsgroups and this link came during a discussion on care for the voice. If you have never seen your vocal folds in action, then you might want to check this page out. While it is aimed at singers, voiceover folk use the same equipment.

http://web.me.com/bdietzler1701/Mr._Dietzlers_MusicPage/VocalCords.html

And then scroll down the page a bit and see how the diaphragm works.