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.
This does not work for me
This can take some getting used to but if you're finding there's little response then instead of gliding your finger around the outer rim, try doing smaller circles in the black space just outside of the motor but inside the white semi-circle ridge - as demonstrated in the video.