[Flush]
This deletes the records of locations of frequently used fonts. These records would make start-up of dviout faster (cf.-Fkeep:).
Note: These font records are also deleted when the -TEXPK: or -TEXROOT: parameters are changed, when the record format version is different, or when font files are moved.
Probably you do not have to use this [Flush] button so often but when the setting of European Language TrueType Fonts is changed, it is recommended to push this button.