Mordhau, otherwise known as a murder-strike, might seem like a made up word if you're not familiar with the term. It's what the guy on the right is doing in this picture. Coincidentally, the person on the left is holding the sword in a half-sword grip. Of course Mordhau (the game) features the murder-strike technique prominently, but it also features other alternate weapon modes. One such is half-swording, like in the picture, others are more subtle, like flipping the side of a war hammer, or gripping a polearm further up the shaft. There are various reasons to utilize these alternate modes, ranging from different damage types (e.g. better suited to defeating plate armor) to shortening the reach of the weapon while increasing leverage -- in other words making attacks faster.
Flipping the weapon upside down seems easy enough, but the opponent will see it from a mile away whilst you approach with that awkward grip! A real swordsman knows how to change grips during followup attacks and throw his opponents off, leaving them clueless (and possibly dismembered) in the process.
And that's how it's done! Another feature making a cameo in this video is a feint, which is the attack that turns into a flourish. The player in this case was quick enough to realize it was a feint and punished the opponent, were he a bit panicky, he might've fallen for it and attempted a parry, which would allow his opponent to hit him with the follow-up.
Until next time!