How Do We Know Jiu-Jitsu Works?
Jiu-jitsu is science, not magic.
What is science, and why is it distinct from magic (however magical scientific findings might seem)? The scientific method is based on observable reality, where you experiment, observe the results, develop an understanding which you refine over time, and repeat. Magical thinking is the opposite: you draw conclusions that you can’t prove. I ate ice cream for lunch, and then it rained later: I should eat ice cream when the garden needs watering. Or, as we hear all too often in the context of martial arts: my system is so deadly we can’t spar, or I’d hurt you.
What’s the best way to determine whether a fighting system is effective? Science. Take a practitioner of that system. Put them in a controlled environment (i.e., a cage) where there are very few variables (i.e. rules), see what happens, and then repeat over and over.
The early Ultimate Fighting Championships were an extended experiment, and a very valuable one.