If the jumper was marked "JEN" it would control whether you adjust FSB, etc from Bios, or via the DIP Switch(es). If it was marked "VIO," it would set your VIO to "3.3v" or 3.45v, or such like.
*Caveat* I've never had an Asus mainboard that wasn't a 440BX, so I can't tell you definitively what this jumper does on your Via chipsetted mobo!