I don’t know what he actually said vs what the translation read, but he explicitly mentions/emphasizes the loose grip, when he’s showing the “make this cut, then make this cut” movements.
The ONLY times I’ve seen people doing that, were ‘knife ballet’ experts who not only had zero experience actually fighting with knives, they had zero experience with even test cutting anything besides air.
I think I’ve mentioned before, but I’m one of the small number of weirdos who’s actually trained with weapons, from various sword styles, to knife fighting, and we sparred at full speed with the rubber trainer knives, made of closed cell foam, with felt edges. They were sturdy enough, that they could and did still leave marks and bruises with solid contact, but flexed enough to avoid serious injury. The felt edges were marked with red lipstick, so you could easily see where someone was scored on.
Real knife fighting isn’t fencing, or knife ballet. It’s pretty much MMA with a sharp, stabby cutting tool. It’s brutal, and fast, and knowing what we know about how long it takes a person to bleed out from even serious wounds, we saw firsthand, the meme that goes, “The loser of a knife fight bleeds out on the street. The winner bleeds out in the ambulance”.
Two fit, fast and aggressive fighters can score critical hits multiple times on each other within seconds. Depending on sparring partners, we might wear open finger gloves, pads and helmets and go heavy contact, where almost anything goes, including throws, knees, elbows because knife fighting isn’t a sport, it’s a no-holds barred situation. Dog Brothers has one of the most well known of these types of schools.
If you slashed at someone with the loose grip he demonstrated, and the blade actually made contact, you’d lose the knife.