Graphics Channels

In the Advanced menu, there's a submenu called "Graphic Channels", where you can see what channels are active, and toggle them individually. There are 16 possible channels, where channel 1 is typically graphics or pictures, and channel 2 contains text.
Easier than that, you can just use the numeric keypad, 1-9 and shift 0-6 (shift adds 10 to the number you press).
For example, on some discs, there are multiple languages in a single song. English may be on channel 2 and French on channel 3. To switch from English to French, just press the 2 and 3 keys at the same time (this toggles one off and the other on).
(You can shut off channel 1, but thats not really recommended.