The TD-9 contains a feature that lets you change various functions by hitting a pad that's connected to the AUX trigger input on the side panel. For example, you can hit a pad and have it change the drum kits in the TD-9 for you.
This feature is referred to "Pad Switch." Here's how to assign the AUX pad to a Pad Switch function.
1. Press SETUP.
2. Press F3(UTIL).
3. Cursor to Pad Switch ( Aux Head or Rim).
4. Turn the VALUE dial to choose the desired function (KIT INC, KIT DEC, Song Start, Song Stop, Off).
5. When you're finished, press EXIT. The new setting is saved automatically.
Now, with a pad connected to the AUX trigger input, you'll be able to control the function you chose in step 4.