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Only one stick moves.
Hold one stick like a pencil, that one is still.
Put the other one along your pointer finger and just use it to press food against the stationary one.
EDIT:
After you have the stationary one in the same place as a pencil, it rests inside your thumb and on your little and ring fingers. You use the other two fingers to guide the moving stick with the tip of your thumb holding in place.
You can see it best here:
http://www.japan-guide.com/e/e2039.html