Palm
If you want to cover a Viking's mouth with your hand, you have to use your palm. Why the palm? Well, if you do it knuckles-to-mouth, you're sort of just punching the Viking… and you don't want a punch a Viking. That's why you use the palm.
しょう
てのひら
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This Viking-touching technique of using the palm was developed by the Shougun (しょう). The Shogun has to deal with a lot of loudmouthed Vikings, so he's learned to use his palm to cover their mouths.