Help needed for Breathing teqnique 2:BREATH OF FIRE

[nikko.kami]

I'm unsure of how to do this exercise, or if i'm doing it right
this is what i don't understand:
"Contract your abdominal muscles, forcefully, creating an exhale, by sucking your stomach in real hard and fast. Immediately relax and air will enter the top part of your lungs by itself."
it really just feels like i'm sucking in my stomach but it does't feel like theres a difference between that and normal breathing.
Heres the link to it
http://www.angelfire.com/empire/serpent ... ng666.html

Thanks! and HS!