Increases the speed with which you get up from a prone position after succumbing to one of the many skills or spells that inflict it, which tend to require either a reflex save or a strength/dexterity saving throw (whichever is higher). (This includes trip, a giant's pound special, the grease spell, etc.)
Also used in hidden rolls against Warforged Titan's AOE knockdown and knockback effects. Success will not only allow you to stand up faster, but can prevent you from being knocked around or down at all.
The feat Greater Single Weapon Fighting requires 7 trained levels of Balance.