Too easy, you start at a certain level based on your age, resting HR (Heart Rate)and other factors.
When the music starts there is a count down and you step, step, up..step, step, down on a set of steps in the testing room.
Up, two, three. Down, two, three.
Your feet have to be fully on the tread and you have to keep pace with the music.
Each stage is the same length of time.
After 3 miutes (maybe 2, can't remember exactly) the PSP staff check your pulse.
If you're HR is good you get to go faster, if you're maxed out you have to stop.
Based on your HR they figure your O2 max.