The "proper" term for Darling's flinch is a stee-ball
. As mentioned earlier, it is the result of poor timing or not tracking the ball all the way into the catcher's glove. Unfortunately, it happened at a critical time and he got a lot of unwanted air time.
With that said, he absolutely made the right call because the pitch was down. While it didn't look pretty, ultimately, he got the call right - which is what MLB wants. It happens to all of us from time to time and big league umpires are no exception. The difference is... when it happens to us, very few people know about it. When it happens at Darling's level, 50,000 fans at the stadium see it and millions more on TV witness it over and over again.