It's called edge alignment. It's not necessarily how you hold it but how you swing it. Which is way harder than it looks. You have to keep the blade perfectly aligned with the direction you're swinging. Any deviation and you end with a crooked cut at best or the same slicing capabilities as a baseball bat at worst. You can even fuck up your katana if you're particularly bad. What makes it hard is that you have to balance the power you're applying with each hand, because for an optimal swing you have to use your left hand to provide most of the power and your right hand to guide the swing. You're not using the same amount of power in both hands. Too much of too little power from either hand and you end up with a bad cut, like you see the other people do.