This is an interesting subject to me as for years a friend and I had been debating how this was done. Often there are two operations to fire something up, a modern propane blowtorch for example would firstly need the gas would be turned on and then ignited from a separate source; usually a piezoelectric igniter. In the elevator sequence we only see Ripley turn something on the bottom of the flamer barrel. Now we did think that this could have been a double-action affair - except that it's rare to have such a switch on any kind of related equipment and it would also have to be designed and built into the body of the prop. Not impossible of course but it would have had to be built into the internals of the prop before it had the dressing or shell put over it.
I was lucky enough to see an original, screen-used flamer many years ago and took some photos of it, including the part that Ripley uses to turn it on. Here's a photo:
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valve.jpg [ 110.55 KiB | Viewed 8007 times ]
It's simply a threaded screw type thing which just turns - there's no click or ignition facility on the flamer at all. It's purpose is a crude regulator-valve to adjust the flow of gas going to the pilot-light burner. Also in it's furthest 'in' position it would stop the flow of gas completely. What I suspect happened in the film was that the pilot-light on the flamer was running, then Sigourney turned it off. Run the film backwards and hey presto - it looks like she's turning the flame on.
To corroborate this there is a behind-the-scenes clip of Sigourney using the flamer on the eggs in the final part of the film. As 'cut' is called a technician runs into shot to take the flamer from her, and while doing so reaches underneath the barrel to this knob and turns it. The pilot light then goes out.
Here's a short video clip of the three sequences:
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flamer.mov [2.07 MiB]
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Harry