go to 18 minutes and 13 seconds-how do  you do it with GIMP?

if you don't want to watch to 18 minutes and 13 seconds  here is the set up:

He took multiple photo of the exact same scene holding a flash over various sections of his scene to shine light on the area. (he was in the shot) Then he put the photos on different layers.... I think I know.... he then removed the pixels from the layer on top which were darker.Is that correct?