Use your finger to create an indentation in the napkin. While it appears to spectators that you are simply creating an indentation, you’re actually creating a vertical channel by pushing down with your finger and against your other hand, and causing the material of the napkin to move into your hand. This is the secret to the trick.
The napkin appears normal from the front, but you now have a tube to pass the knife through. Note that the channel in the picture has been exaggerated for demonstration purposes.
