I don't have a system icon, lol.
Au contraire It just was described in short. Here is the long part:
When on the parameter page, you see fields for all the parameters and in the lower part of the window is a list of icons. They represent the expressions related the repective objects. When entering one of the fields, you may either type by hand (and then use, what lucid wrote) or you select the system icon via doubleclick, then in the new window with the system expressions select the system tab and afterwards "Get global variable". Click on Finish, and you will see a "global('Variable name')" inserted into the parameters field. Now just replace 'Variable name' with 'FASTMODE'.
In this case, it doesn't make much sense, because it's faster to type it, but you do not always know exactly the syntax of an expression, and there it comes in handy.