This is the (inside) pocket bracelet "Marty" Michael J. Fox vapes in Back to The Future: the Hoverboard Scene. (Watch his pocket "shift".)
Step 1: Charging the Back to the Future Transponder at Night
A macroscopic electrostatic hexadecimal "toothbrush" future transponder under city limited timed night light for charging time travel.
Step 2: Transponder Run
Marty wears this shifty pocket transponder to time travel throughout the movie, behind scenes.