We use both actually. Just as you don't use an erect palm to someone's abdomen, we don't use a vertical fist there either (very unnatural and too much of a bend in the wrist for either).
We follow the rule that as the hand moves down the torso (whether it a palm or fist) is turns. This movement for us starts just about the midline for both palms and fist. Once at the lower abdomen it is a horizontal fist. The movement for the fist is in towards center and the palm is out away from center as it moves downward.
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