To increase the intensity of the vibration, touch your finger at any point on the edge of doppel’s face.
Holding your finger to the edge of doppel’s face, sweep clockwise all the way around the outside edge of the face.
Once you have made one complete circle, continue to sweep clockwise until you reach an intensity you like.
If you overshoot, just change direction (keeping your finger on the face) to adjust.
To decrease the intensity of the vibration, follow the steps above but use an anti-clockwise motion.