Flightless Butterfly
Why does it cut through something horizontally better than it does vertically?
I'd say it's moreso the technique than the actual sword. It's probably a lot easier for these guys to put the proper weight and momentum into the horizontal swing, whereas with vertical swings I always hear about needing strong abdominal muscles.
Then again, this is going by Japanese swordplay, though I figure there are at least some similarities with regard to full body integration.
What made me chuckle about that commercial was the obvious cut they made when the guy did chin-ups with the sword, bending the blade, but couldn't show the blade after he was done doing it without cutting away. Then again, considering I've seen them do the bend trick with a katana and it didn't warp, I'm sure it's a similar deal.