I think the trick lies in the way Freddy Franz used, rather than "translated" the Hebrew word "yatsah" which occurs here. The word in its broadest sense means "to depart", that is, to leave one area and to transfer to another. That Franz knew the meaning of this word is evident from the way he "translated" such verses as Ex 16:1. Here, as in other verses, he used the word "depart".
But at Gen 35:18, the translation used was ambiguous enough to mean something rather different from an actual transfer from one place to another. In using "going out" as his preferred option, Franz allowed for a conclusion that more aptly referred to a light bulb, rather a departing to another location. When one turns a light off, or as someone mentioned above, blows a candle out, it "goes out" thus implying an "extinguishing" rather than a "departing". That is what Franz conveniently suggested Gen 35:18 meant.
In many ways, Franz used the notion of the NW"T" as being a literal translation to create a version that could, in many verses that are awkward to Watchtower theology, be so excruciatingly literal as to be totally unintelligible.