The claim makes sense in the statement’s original context, which was probably trying to articulate the dangers of force multipliers, the knife as an ambush weapon, or a self-defense professional discussing knives used in muggings against unarmed combatants. In every one of these examples, it’s a (admittedly bad) metaphor trying to illustrate a concept that can be difficult for individuals with limited backgrounds to understand.
What the claim isn’t is a blanket declaration of fact, because in that context it makes no sense at all. However, someone makes the statement, someone else parrots it, and we’re off to the races. Now, you’re here with a statement ridiculous on its face because it removes all the conditions and is basically saying knives are magic.
The knife is a force multiplier in hand to hand combat, making the individual who carries one far more dangerous than one without. The 10 years is trying to convey that the knife, especially as its one of the most common weapons encountered in a modern, urban environment, is a very dangerous weapon that has killed many experienced individuals.
Getting your students to grasp how dangerous (clear, and present, will kill you even in inexperienced hands) can be very difficult due to how the knife is often disregarded in popular culture or written off as a weapon for gang members or fantasy rogues.
The actual example is this: “You have two martial artists of equivalent skill who have both trained for ten years, but one of them has a knife. Add an additional ten years to the guy with the knife, and that’s what you’d be facing.”
This is not adding a literal 10 years of training, this someone trying describing the additional dangers presented by a force multiplier. What this example doesn’t mean is that a person without any training at all has ten years of martial arts training, will magically turn you into a martial arts master, or that the knife makes up for training you don’t have.
Someone utilizing the knife as an ambush weapon in a mugging can stab you multiple times to the point where you will bleed out and die on the street.
Someone with actual training in using the knife will kill you much faster than the mugger.
Someone with no martial training trying to use a knife for self-defense can brandish it, at best. If the threat of violence doesn’t work, they won’t know what else to do with it other than swing wildly. Swinging wildly will risk the blade being taken away.
The knife is not a replacement for martial training, it adds an additional force bonus to what you already possess, and makes you more dangerous in hand to hand combat; especially against an unarmed opponent.
Try to remember, for the most part, martial combat doesn’t have universal rules. There’s a lot of great advice out there, but everything is contextual. Everything is conditional, there’s always an exception, and those conditions and exceptions when utilized appropriately significantly change the field of battle.
Here are some basic examples:
Size doesn’t matter except when ground fighting.
Ground fighting, when lying on the ground, like grappling has to deal with the full weight of gravity. Here, weight, height, limb length, and leverage all play a significant role that they don’t while standing.
The gun is king except in close quarters, when you aren’t given time to draw.
There’s a double whammy for this one. If you don’t have time to draw or your opponent is past the gun, the gun is not king, which is where the eight foot or two meter rule (these are different distances) comes in. However, those rules only apply to Weaver/Teacup/most normal stances, and don’t apply to those individuals trained for close quarters shooting. (Like CAR.)
You might think some of these are obvious, but there’s often a rush to generalize information so its easier to understand. Generalizations can impede your understanding and, ultimately, take statements out of context. However, it is easier to generalize than itemize all the situations where a statement isn’t accurate.