This is 100% posturing with zero applicability to a real knife fight.

First, the attacker in these videos is always strangely passive, letting go of the knife/gun with a slap on the wrist. In a real situation, when you move your hands the attacker also moves his, and while you may be able to get him to drop the knife, most likely you'll end up with a bad cut, on your throat, arms, torso, or elsewhere - remember, he's got the grippy end, you're facing the pointy end. It's not at all easy to get someone to drop something just by slapping their arms down, or twisting, or whatever. A hard blow with a rod or stick can do it, but not much else, at least not something you can do from that starting position (knife at your throat, you with your hands up).

Second, only in the movies can you win a knife fight using a reverse grip. With a reverse grip, there is only one good strike that can be made, the downward stab. It sucks at everything else - your reach is a lot shorter, you can't do any of the cuts that you can with the correct grip. Yes, you can put more force into a downward scything blow, but when you have a knife you don't need force, you need reach, skill, speed, and agility. If a man comes at you with a knife held in a reverse grip, you can defeat him by simply holding your arm out straight with your knife pointing forward, so that he impales himself on your blade before his blade can reach your body.

Third, the attackers in these videos always make things as easy as possible for the defender. In the third "attack" (00:07), note how the attacker uses a wide roundhouse sweep of his arm to attack, instead of driving straight forward as would happen in real life. This gives the defender the opportunity to block while countering with his own attack...

...which sucks. Note how he does an awkward face/upper arm slash at 00:10, then another awkward backhanded stab to the abdomen, followed by another slash at the neck, when one simple thrust to the abdomen with the knife held the right way would have done the job and ended the fight.

TL,DR: This is pure posturing done to impress gullible wannabe badasses. In real life, if attacked with a knife, shoot your attacker, be wearing an Iron Man suit, run, have a longer weapon, or have a knife of your own, in that order of preference.