Seeing this listed under Gamespots's hints got me wondering just what the heck this is. Is it a DVD style commentary? If so, how is that even possible?
Yup, you can play each level with full running commentary from the designers who discuss design aspects of the specific level you are playing. I found it better to pause the level and listen that way.
Its actually a neat feature. The only thing is, there are 16 commentaries (15 missions plus the training mission) so to hear them all, you need to sit through a lot of talking.