Rikku’s Overdrive is called Mix. It allows her to select two items from the inventory which are combined to create a new and usually more powerful item. There is a huge list of combinations that can be created when mixing items. The Final Fantasy Wikia site has compiled an excellent list which can be found here:
Often referenced within this strategy guide are the Super Mighty G and Hyper Mighty G. You should learn some of the common items that can be used to create these two status generating items.
Her most powerful Overdrive is called Trio of 9999 which can be used relatively early in the game to make some of the earlier fights very trivial. It causes all attacks and spells to deal either 9999 damage or heal for 9999.
