The lower right of the top screen shows your “Drop Gauge.” This depletes with time, and when it reaches zero, your character is automatically switched between Sora and Riku, a process known as “Drop.”

During the change, the Drop Prizes, which you get from defeating enemies, and some other items are converted into points. These points can be used to temporarily raise the parameters of the incoming character, or obtain new commands and items.

You can also trigger the Drops automatically. However, it is not possible to progress through the game as just Sora or just Riku.

(Source: andriasang.com)