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.
Highly opinionated but out of touch with the current MMA scene.
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