Custom Scale Plarail

how there are 4 speeds and how to change them are the first questions that come to my mind.

By "four speeds", they mean there are four different ratios to choose from when dis/assembling the gearbox (each one obviously requiring a different configuration). I don't get how Hirox1955 made his engines two-speed (not four) with only a SPDT switch and no extra component that would affect the voltage. Unless he just took the gearbox apart and rebuilt it before showing the higher/lower speed.