Left clicking on the toggle will set the value to Yes, and the toggle is no longer grayed out.
Left clicking on the toggle again will change the value to No.
Once the toggle is clicked and an initial value set, you cannot set it back to Null, so, in operation, there are only two values, Yes and No, but you need to be aware that parameter can have that third value, Null, as that can have an impact on your model. For example, suppose the HasToggle parameter is used to drive two filters, that will be used to change the display of Walls in a view.
In the image below, you can see eight Walls in the LEVEL 1 FLOOR PLAN View. Two have had the HasToggle parameter set to Yes (checked), two have had the HasToggle set to No (unchecked) and the other four remain in the initial Null state. The Null Walls are not affected by either filter, because the value of the HasToggle parameter is neither Yes nor No.