not a handbrake
but a diverter valve plus a switch. It was sping-loaded, very much like a regular toggle switch.
Before choosing what to do (heat, wash or spin) you had to slam that thing towards washer or spin dryer compartment, so the drain pump can take the water from either of the two.
In other words:
The 3 knobs were: timer (some minutes), thermostat (up to boiling) and function (heat, heat and agitatte, drain (drain or spin and drain respectively).
So with that large lever to the left = drain pump working for the washer part.
To the right = drain pump for the spinner part and spinner motor "on".