The spinal cord is in the center of mass as well as the large nerves that branch off from it. Of course the heart, but also the aorta and superior and inferior vena cava, the pulmonary arteries and veins and the carotid arteries that go to the neck and head. The liver has large blood vessels that can cause heavy bleeding. A hit to these areas can cause an immediate stop.

Tell your friend to look at the markings in a standard silhouette target. The center and '9' areas are the center of mass on a human torso. As you go farther out from the center of mass the less sure to neutralize an attack.

The upper chest and abdomen have these same structures and can produce a quick 'stop'.

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"He was kinda funny lookin'"