Before you even turn it on watch this video!
Understanding the power levels ...
The fingers tips and lips (for example) are much more sensitive than palm of the hand or the cheek.

1. Place the electrodes on the skin with power = 0.
2. Then slowly increase until you just feel the tingles.
3. At some locations you might not feel anything even at very high power levels.

If that happens BEWARE ...

After some time you might suddenly feel the power (as the electricity begins to travel into the skin) and when this happens remove the device and lower the power.
Then test again, and you won't get a shocking jolt.

This is especially true when working on others.