One wire is hot the other is ground. The difference is which way the current flows. You must have the right type of switch to make this happen. When that switch is centered both wires are probably neutral, neither hot or ground. switch position provides a path to chassis ground via 3 wire from the switch to ground or maybe even a metal body switch is chassis grounded at its mounting.
A chassis ground to the motors exterior case isn't required although there probably is a chassis ground path through the motors mounting hardware.
