In the "Custom Filters..." dialog, the filters is said to "hiding games that match the filters applied".
But the Description filter works the other way around, it SHOWS the games matching the filter. And for all folders, not just the one.
In my case, I want to hide mahjong games, card games, casino and sports games.
I suggest having two textboxes, one "Include games matching these string" and one "Exclude..."
And also tell the user that it's a global filter.