Thanks @gillianpearce, I am glad it's not a nerds-only read ☺

There are two moments when I need darkness:

• When I make a print: For this I project the negative on the light sensitive paper. That moment (exposure) the paper is kind of out in the open. That's when I need a dark room.
• When developing film: I use a 'dark bag' to put the films into the drum. It's a light proof bag with two sleeves to put my arms in. This is just enough space to move the films from the camera/holders into the drum.
  Maybe this will shed some more light on the concept:
  After projecting the negative on the paper, I use an automatic processor to develop the print. But I could also put the paper into this drum and develop it there...