Due to a nearly complete lack of documentation for the material system, much of what I know is from hypothesizing how something works and then testing the hypothesis. However, this somewhat failed me in determining what the Translucence channel does, or more specifically how it was intended to be used. Largely it does act like the ambient or alt_diffuse in that it just seems to emit whatever you set it to or plug into it. But that's not its purpose. Unlike those others, this channel reverses the normals used in Diffuse lighting calculations of a dedicated Diffuse node. It is so that you can get the back-lit paper-thin effect from either side. It is only useful for one-sided objects like your leaf. It does not help in any way with solid objects if shadows are enabled as they should be. In order to get it to do its back-lit paper-thin effect, you must plug in a Diffuse node, with the Normals_Forward option checked, and you must not send the data from that diffuse node to any of the other channels in any way. It must be dedicated to the Translucence channel.

I must add that it is buggy as hell. As far as I can see it is doing its thing as designed for directional lights and a dedicated Diffuse node, but for IBL or IDL, it doesn't behave like real translucence under those lighting conditions. I still advise not to use it, unless you first study it completely and understand how it will burn you if you go where it cannot go.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)