FTP Test Question

I know virtually nothing about FTP testing etc so the instructions are useful but confusing.

As far as I am aware I should choose a middle cog on my cassette and just pedal until I can't pedal any more. However the instructions mentions changing gears. When does this take place - in the warm-up section? Or do I change down once my cadence drops below 40 so I can artificially boost my cadence?

Thanks

Steve