Header explanation? Please

Ok I found the episode of "Two Guys Garage" that had the exhaust flow breakdown:
[ame="http://www.youtube.com/watch?v=n8VlynaUVO8"]http://www.youtube.com/watch?v=n8VlynaUVO8[/ame]

At 2:13 he goes into explanation. @ 7:07 he talks about the "pulses" that make the tubes longer or shorter.