I've got this answer from Cristal River Ltd, which I think answers everything, my misundarstanding was that I thougt it was a light, not some luminous thing.

My mistake, I'm still banging my head against the wall.

So here is the answer:
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Hi Thorarinn,

Good to hear from you and thank you for selecting one of our products.
You do not have to do anything - they glow all the time - just take them
into a dark room and you will see the soft glow in the darkness. They have a
life of around ten years plus - longer than I manage to keep my flies on the
end of my leader

In terms of tying, as you mention, you can add extra strength to the light
by shrink wrapping them or just tye them in as they are. Normally they are
tyed in as a tail or as part of the body with either dubbing or tinsel
either side of the light.

They are mainly used in the UK for Sea Trout at night or on dull days for
salmon and sea trout. However I do know that a number of still water fishers
have been using them on heavy sunk lines for deep lake fishing - with great
success.

Please let me know if I can be of any further assistance.
Best regards
Paul
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So now you know.
I often get brilliant ideas when tying and the flies are brilliant until I try them in the water, mabe this is one of those ideas.
But anyhow, it's fun to experiment.


Regards,
Thorarinn