Here is Peter Gorman's description:

Nu Nu is a snuff made from toasted, crushed and finely powdered mapacho (jungle tobacco) leaves combined with the toasted, crushed and powdered inner bark of the macambo tree--a member of the cacao family.
It is not, like yopo, filled with tryptamines, and isn't psychedelic at all. Nonetheless, at large doses, maybe 14 half-gram bursts blown up your nostrils, there frequently comes a point--after the pain subsides--where you see something like a television screen and across it you'll see all of the animals of the jungle running.
One of those animals--or several of the same species, generally--will fall and die. You concentrate on that one: Look for how it dies, what time of day it is, what part of the jungle they are dying in. Then the next day you go there and wait for the animal to arrive and kill it as you saw the previous day.
The old Matses, the antiguas, say the animal sees the vision of their death as well, and still comes to the spot to die as that is its fate.
The brief pictures of the animals pass quickly and are followed by a general feeling of euphoria that can last 15-30 minutes.
One of the keys to making the nu-nu effective, say the Matses, is that the person blowing the nu-nu is also blowing his spirit and strength into you--not as in giving it away, but as in empowering you with his force as part of the force of the experience. So it's not just how hard you can blow, it's whether you can share yourself while doing it.
And the nu nu is generally, or frequently in my experience, made by two great hunters--one working the leaves, the other the bark--before they're combined to give some of their force to it as well.

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I'm here in Iquitos, going out with PeterG next week. I have some sapo and nu nu I sent back home to the USA.
It's there waiting. If you want some I'll gladly sell anyone some. I get home July 20th 2007