The TD-8 contains a feature called "Pad Switch" which lets you change various parameters by hitting a pad that's connected to the AUX 1/2 jack. For example, you could have two pads connected to the AUX 1/2 jack, and have one pad increment through the drum kits, and the other pad decrement through the kits.
Here's how to assign the Pad Switch function to one of the AUX inputs in the TD-8:
1. Press SETUP.
2. Press F3(MENU).
3. Cursor to "SWITCH," and then press F3.
4. Press F2(PAD).
5. Use the Cursor buttons to navigate to the AUX1 setting, and then turn the VALUE dial to set it to the desired parameter:
(KIT# INC, KIT # DEC, SEQ # INC, SEQ # DEC, OFF).
6. Repeat Step 5 for AUX 2 if desired.
7. Press the KIT button when you're finished. The new settings are saved automatically.