The TD-15 contains a feature called Pad Control which lets you control various functions by hitting a pad that's connected to the AUX input.
For example, you could hit the head of the pad to increment through the Kits, or hit the rim of the pad to decrement trough the kits.
Here's how assign this feature.
1. Connect a pad to the AUX input on the side panel of the TD-15.
2. Press MENU.
3. Press the cursor down button until "SYSTEM" shows in the right side of the display.
4. Press the function 3 (F3) button.
5. Press cursor down to choose "PAD CONTROL."
6. Press F3(ENTER).
7. Turn the dial to choose the desired function for the head of the pad - (OFF, KIT +, KIT -, SONG PLAY, SONG STOP, SONG PLAY/STOP, TAP TEMPO).
8. Cursor down and turn the dial to choose the desired function for the rim of the pad.
9. Press EXIT when you're finished. The new settings are saved automatically.