The manual states:
The use of this function is recommended when placing a very high
quantity of food in the refrigerator and freezer compartments. Using
Fast Cool function it is possible to increase the cooling capacity in
the refrigerator and freezer compartments. Note: Fast Cooling function
should be also switched ON before placing fresh food to be frozen in
the freezer compartment, in order to maximize the freezing capacity.
These functions usually turn off after about 3 hours or when the set temperature is reached again. As the recommendation is to turn it on before stocking, it's safe to assume that it's a simple time-based "heavy duty" mode. As there is only one button for the fridge and the freezer, it has only one cooling circuit. So you turn it on when stocking the fridge or the freezer and so one compartment is "fast cooled" in vain.
The purpose is not to further lower the temperature, this is merely a side effect. It would be kinda silly to lower the temperature and then open the door, let room temperature in while stocking and let all the cooling effort dissipate. I would give it ten minutes for the cooling circuit to prepare and then start the stocking.