Oh, it's about the repair of an LED light source. You have to come up with that first, given the title.

The LEDs are like old-style Christmas lights, wired in series, so that if one burnt out, the whole string of lights didn''t work until the bad bulb was replaced, So he sequentially used pliers to conduct over each LED [the small black rectangle] with the light bulb on until it lit That electrically bypassed the bad LED with the conductive pliers, completing the circuit again, electricity flowed, and the LED's lit. Now he'd found the bad LED. He then used the conductive carbon dust from the pencil, as powder, to make a conductive path over the dead LED, bypassing it electrically, completing the circuit, and the light worked ! The glue just locked the new conductive bridge in place. Cool ...

@@richardwellons5138 Thanks for the explanation. It doesn't exactly look like Christmas tree lighting and the effort involved in checking all the light sources would be very high in some cases. By the way, the title has now been changed in the German translation.

N'est-ce pas un peu problématique de shunter une LED (par surcharge des autres éventuellement ...)