Black "Button" on Lid.
The top of the Gyrator has a diaphragm. When the washer fills the Gyrator Cap rises due to the air pressure in the Gyrator column raising the cap to press the button. Inside the lid, there is an adjustable mechanism with Mercury switches or switch for the water level.
When the Cap rises to press the Black Button, the machine begins to agitate.
I believe it also serves as the "Safety Don't you dare watch the cycle" cut off.