Earlier quoted context omitted.
My theory is that this isn't a pun, but rather a term that's no longer in use. I seem to remember that some early elevator systems had two connected shafts and two cars, instead of one car and a counterweight like modern elevators. In this arrangement, cars go in a loop around a pulley at the top and bottom, always in the same direction. Thus, one shaft is the "up" shaft and one is the "down" shaft. This image depict…
> some early elevator systems E.g. http://en.wikipedia.org/wiki/Paternoster
(And "descenseur" seems to be an actual French word according to http://fr.wiktionary.org/wiki/descenseur, so much for showing off and knowing my native tongue.)