This will stop anybody that would normally be stopped by pain. If a bully feels pain, sees his victim fighting back, or feels the odds are now against them, there is a high to certain chance the bully will attempt to retreat. It doesn’t work on those that aren’t stopped by such things because they have chosen not to be. Pain, is after all, a stimuli that still gives you a choice. It’s just a very persuasive argument not to do that which causes more pain.
 
Knives use the same principles to cause damage that anything else uses. Take a blunt force impact with bone or wood as an example. The difference is that except of doing damage with a sharp edge, you do it with blunt force trauma. Instead of bleeding from the skin to the outside, you get internal bleeding. Instead of a nicked or cut portion of the human anatomy, you have a crushed anatomical feature that is now just as broken as if a knife stabbed it or cut it. The same principles apply in either case. Kinetic force is applied to a target, translated, and then set up so that the energies are all expended in the target. For a knife, the target is what the edge or point hits, which being very small translates enormous kinetic force from the handle/user, thus creating a break in the target material. Blunt force trauma from any object or human body part uses the same principles, except spreading the force over a broader area. Thus it requires greater force to penetrate as deep as the knife would have with lesser force. The force will translate into the body as a shock wave, but not break the skin. Usually because the compression limit of skin is better than your heart or liver’s ability to compress in on itself. Not to mention hematoma for the brain.
 
Humans beings figured this out either through experimentation or science. They started refining these things, like adding weight to the tip of a big sword to make it cut through armor better. They changed the curve and shape of a steel sword to make its utility different. They changed the composition of steel to make edges maintain sharpness and to have a better edge in general. They added a huge ball of metal spikes to a club, thus crushing plate armor (translating enough force to dent the armor and rupture internal soft organs) rather than attempting to cut through very strong steel.
 

The thing is, now that we live in a decadent society, we live under limits. Much of the tools we could wield, are now restricted. Whether it be the firearm or the nuclear bomb. You are prevented from utilizing it except under specific scenarios. So that engenders an instinctual need to find replacements that can be used without such limitations. But at the same time, those replacements are never going to be as good as what humanity originally intended for war because anything intended for war is restricted. So we’re left with our original ability, the human mind’s ability to make a tool out of anything in order to solve the problem of a living threat. And our first immediate tool is our body, one step away from the brain. Unlike a credit card or a key, you’ll never lose your body, unless a limb gets amputated. But then I think you have more serious problems than you can’t use that limb anymore.