Who knows the working principle of digital meridian physiotherapy instrument, and why does it feel like massage? It didn't move by itself.

The instrument didn't move, and neither did you. It is the current that passes through your body and makes you feel it moving. The current is introduced into the body mainly by two electrode plates, and something will collide with you in the nearest place of the electrode plates. When the current is connected, the contact part will creep, which is due to muscle twitching caused by excessive current. Its physical therapy principle also works through electricity.