As feets said, his shop guy thought that a voltage difference in the trigger line might cause it to blow slower. not variable but 2 speeds.

I hooked up a balast resister in line to the trigger line and tried.
At first I thought it didn't make a difference, but later that day I realized the fan had not spun up to high speeds. I have not tried it again to verify though.

Still very fast at "low" speed, so not sure there is a benefit to the extra wiring required.