How realistic is stopping a knife from killing you by grabbing the blade with your hands?
Kind of. It’s realistic in the sense that it can and does happen. At the same time, it probably won’t save your life. Knife wounds to the palms, (called, “defensive wounds,”) are fairly common when someone has been attacked by a knife wielding opponent. Usually, what happens is they’ll attempt to block the knife by putting up their hands, palms out, and their palms and fingers will take the initial assault. That I’m most familiar with the term from autopsies should say a lot about how well this usually works out for the victim.
If you’re dealing with a situation, where someone’s trying to stab you and your only option is to catch the blade with your hand, it is better than dying. However, it is also a very temporary solution, and one you can’t repeat after using. It’s also, probably, not your best option.
When you bleed, your body is trying to do two things; first clean the wound and expel any foreign objects in it, then seal the wound over to allow the tissue to heal. Fresh blood is aggravatingly slick. Once exposed to oxygen, blood becomes tacky and coagulates over the course of a few minutes. (Specific clotting times vary based on a number of factors. For example: if your character is an alcoholic, their blood’s ability to clot will be severely impaired.) It only remains tacky for a few minutes, and will then harden into a solid mass, so the window here is fairly narrow.
When you take a knife to the hand, you’re going to bleed all over your hand. That means your hands will get slick, and have a harder time gripping the blade. This is before you consider the part where your hand is actually getting cut to pieces. Eventually the blood will clot (whether you survive long enough to see this or not), at which point gripping the blade would become easier, but that’s not a realistic consideration because the fight won’t last long enough to get there.
As I’ve said before, your body functions on a kind of pulley system. Your muscles pull on tendons which in turn tense against your skeleton, causing your limbs to move. When you start cutting tendons, the pulley system starts to break down. Some of the most delicate pieces of this system are in your hands and feet. Start carving those apart, and your hand will not work. This isn’t an, “oh, I can force my way through on sheer willpower,” situation. The mechanical components critical to making your hands work will be damaged or destroyed. The spirit may be willing, but the flesh has been turned into butterflyed steak. Catching a knife with your hand will stop that strike, but it means your hand will not work again. Yes, if you survive, it can be repaired surgically, but that’s not going to keep you alive.
The better option, if you have sufficient manual dexterity to catch the blade is to catch your opponent’s wrist instead. Again, this isn’t a great position to be in, and wrist grabs are some of the weakest and riskiest holds, but it is far better than trying to grab their knife. Your arm or hand might get nicked by the blade, but that is vastly preferable to taking a direct blade to the hand. Going for the wrist is a legitimate strategy and a part of some knife fighting doctrine. Granted, your best option would be to maintain distance, and never let a knife wielder get close enough to attack, but that’s not always a practical option.