You should experiment with this yourself, but I usually start with my nostrils just a little bit away from the piles of snuff and, as I sniff, I move my nose onto the surface of my hand while taking the tobacco into my nose. A lateral movement of your hand just as you finish off the sniff tends to clean up all the tobacco, from both your hand and your nose.
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The first of the locations you can use on the back of your hand, is what is referred to as the "anatomical snuffbox" (see Image 1 above). This area is a depression that forms behind your thumb when you stretch your thumb out away from your hand. It can vary in depth, and thus usefulness in snuff-taking, from person to person. Another factor that can stand in the usefulness of this location, is hair. If you have hairy arms, the anatomical snuffbox may feature body hair which can get in the way of a good snuff-taking. But, so common was the use of this area of the hand for snuff-taking, that it became a part of anitomical terminology.
My personal preference, is the flat surface of the side of the hand between the thumb and the wrist (see Image 2 above). This is a wide flat surface, usually free of body hair, and can be used with the hand in a fairly natural and comfortable position.
The last location I'll describe here is the webbing between the thumb and index finger (see Image 3 above). If you move your thumb away from your index finger, this webbing forms a pocket that can be used to hold snuff. Essentially your thumb goes along your face to one side of you nose with your index finger along your face on the other side of your nose. This brings the snuff in the webbing between your thumb and index finger under your nose. This is not a particularly common location for taking snuff off the back of your hand, but it is occasionally done.
Again, the point here is not to define a "correct" or "approved" location for snuff-taking off the back of your hand, but to instead describe some of the locations that are used.
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