Here it is an extract of the User Manual about the folder name filter:
"The filter by file name mask applies only to files. Sometimes you may want to search only under some folders, say, temp folders. You don't know where they are and at which nesting level on you media device, but you know they are there, and possibly more than one.
In this case you can use a folder name mask filter. Folder name mask filters are the same as file name mask filters, except they apply to folders. To identify a pattern as a folder mask pattern, simply prepend it by the '\' (backslash) char. The backslash char doesn't have the meaning of "root folder", it is simply a switch. It has been choosed as "switch" char because it is forbidden in file and folder names (it is a path separator), hence it cannot exist into an element name.
When you apply a folder name mask filter, only files whose path contains a matching folder are inluded. Moreover, folders which name matches the mask are hilighted with the BOLD style in their name. This makes easy to understand why stuff has been included by the filter. If, on the other hand, you are using an exclusion filter (by prepending the whole pattern string by the negation pipe (|) char), excluded folders are not hilighted just because they will not be displayed.
Example: by typing \temp every file under a temp folder (at any nesting level) is included. For example, C:\Windows\temp\data\abc.txt would be included.
Example: on the countrary, by typing |\temp every file under a temp folder (at any nesting level) is excluded.
Hint: You can consider folder and file masks as a two dimensional filter. The first axis is the folder mask, the second axis is the file mask. Both axis work independently. When you apply a folder+file mask you are defining some kind of "coordinate" in a 2D space. You first "cut" the directory tree structure and isolate the searched folders, then examine the wanted file names by the other mask."
...and, on the online quick tip window...
By folder name
Same as by file name, but prepend with '\' (backslash) (|).
The folder mask considers files whose path contains elements
that match the mask at any nesting level.
Matching folders are renderer with BOLD font.
The mask itself cannot include the '\' char.
Example: \temp will include only files whose path contain
at least a 'temp' folder.
For example, C:\windows\temp\data\abc.txt is ok
Example: |\progr* will exclude files whose path contain
at least a 'progr*' folder.
The starting backslash char does not mean 'the root', it has
been choosed as switch because it is a forbidden char
in files and folders name.
(Note: Folder masks are slower than file masks)
Honestly, i'm still looking for the "magic words" that make the user understand the concept with less text. Sometimes i've a bit of difficulty because i'm not english native. Hints are welcomed!