It's what's called a variable water level control, with a range of choices between minimum and maximum vs. a control with specific levels such as Low, Med, and High.
Photo one is a variable switch. The cam is a smooth eccentric arc from minimum to maximum (other than typical tiny notches on it to avoid slippage).
Photo two is a three level switch with deeper notches on the cam for each discrete level.
Reset is used to increase the level after agitation starts. Turn the control to Reset (agitation stops, fill resumes), and back to a desired level that's higher than the initially selected level. It is not necessary to turn the knob to Reset for selecting a level for the next load before starting the machine, or for changing the level after starting a load if agitation hasn't yet begun.
A Reset function is needed on variable switches because the smooth arc of the cam doesn't otherwise have a way to force the pressure contacts apart to restart filling. The deeper notches on a fixed-level switch fully depress the actuating lever to reset the pressure contacts each time it's turned from one position to another.
