If you shift it momentarily gets easier while the resistance ramps back up (1-3 seconds depending on trainer) then shift again, and again, and again. You can probably extend the test 30 seconds if you run through the whole range, granted your power would momentarily fall with each shift so you’re not really gaining anything, but it’s just a poor idea to try and justify with how much ramp test confusion there is already.
Sit, pedal, when your cadence starts to fall you’re done the test