Here you go.
As Terry said via Matt the horizontal rubber band provides enough tension on the fulcrum to hold the lens in place in the retracted position as it's held at an angle. When the wearer pushes the green lever up it straightens the fulcrum, allowing the lens to drop down aided by the vertical rubber band. The circuit is then made by the large brass strip being 'live' (i.e. connected to the positive terminal of the 9v battery, via the bulbs) coming in to contact with a piece of metal on the lens which is (also via the bulbs of course) connected to the negative terminal of the battery. When activated the circuit is made and the bulbs come on. Simple!
I think in one shot, if you slow it down you can see the lens drop, the bulbs come on then flicker off before turning on again as the lens bounces on the brass bar!
If you want one though I'd seriously recommend that you stick to a Spat etc. version; I've built a few of these (three if I recall) and it was incredibly fiddly; so much so that I vowed never to make them ever again! Of course at the time they were made in a hurry to be used once or twice and then discarded so didn't need to be robust and reusable beyond a few days.
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Harry