I don't feel like punching that way is as powerful. I think curious1 hit the nail on the head, in that, vertical fist is more pushy and lacks snap. That said, I like to throw vertical-fisted uppercuts sometimes, since the twisting of the fist (or lack there of) doesn't affect the mechanics of the punch at all. It definitely does slip through people's guards better.
That's not always the case. Jack Dempsey was a fan of jabbing with a vertical fist. Then again, he usually accompanied it with a substantial drop step. In which case, the power was from definitely more from the feet up.
Then again, correctly applied physics would increase the force of any punch substantially.
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