The housing of the viscous coupling, onto which the fan-blades are mounted, should, with increasing engine temperature, be more solidly connected to the wheel which, in this case, is driven by the crankshaft.
The air, that flows through the cooler, remains cold. A bimetal spring, which is connected with the ribbed aluminium through heat conduction, does not (yet) bend. Thereby, no viscuos fluid flows from the reservoir to the operations area (on the left). On the contrary, because of the big difference in RPMs, the wheel in the operations area pumps all the fluid out into the reservoir.
If you click on the figure, you'll see that the bimetal (two metals, each with a different heat expansion range) bends more strongly. The pressure plunger moves a little to the left and the (slightly sprung) intermediate disc opens the upper port between the reservoir and the operations area a little bit. The operations area is half-filled, from the outside edge, with viscous fluid.This provides for a certain amount of connection between the driven wheel and the housing. The cooler fan (in our example) runs at about half the RPMs of the crankshaft.
When the bimetal has reached it's maximum curvature, the port between the reservoir (left) and the operations area (right) is completely opened. The viscuos fluid completely fills the operations area. The RPMs are now, give or take a few hundred rotations per minute, roughly the same. 11/10