The analogy I use is pilots training for emergencies. They train specific skills for emergencies, as well as how to apply skills they use for regular flying. They even do some simulations, so they can practice the emergency maneuvers, with the understanding that the simulation is only as real as it can be. Then they fly a lot. When an emergency happens, they quite often (but not always) manage to pull off those emergency maneuvers. Sometimes their judgment is off, and sometimes they lose their cool and mess up. But a lot of the time, they manage just fine the first time they hit heavy wind sheer, an engine goes out, or one of the other situations they trained for. They even do well in some situations they never trained for, at all.
SD physical training is similar. We train specific skills for emergencies, as well as how to apply more basic skills then. We use some simulations to get closer to how those emergencies can occur. We apply the basic skills over and over (sparring, randori, rolling, maybe actual competition). IME (from talking to folks who managed to use their skills), if that basic skill application exists (rather than only drills, especially only cooperative drills), there's a pretty good chance of the skills working when needed. Sometimes they won't work. Sometimes they'll fail because the situation called for different skills. Probably less reliable than pilots' emergency skills (mostly because commercial pilots are a selected population), but still predictable.
The analogy isn't perfect, of course - none ever are. But I think it's appropriate and shows a similar path of development.