Much (but not all) of this discussion about how the Crew rushes other teams hinges on this:
The hitter has to be in the box and attentive to the pitcher by the time the clock reads :08, but of course, they have the option to do so sooner. The pitcher, meanwhile, can't pitch without making eye contact (or, at least, ensuring that the batter is ready), so there's a real sense in which the batter controls the tempo of the at-bat.
Clearly a batter can slow things down by taking all the time they're allowed. But the only way they can speed things up (which is the foundation of Matthew's argument) is by getting ready fast and thereby applying psychological pressure on pitchers to respond to their readiness sooner than they would prefer.
Circumstantial evidence was presented that the Brewers' hitters are in fact rushing pitchers by getting ready fast. But there's no direct evidence of this, such as pitchers admitting to it (which might be the only direct evidence possible).
I'm going to remain a tad skeptical of the theory. The reason is that professional athletes are highly trained and coached, as well as highly competitive, so wouldn't teams see what the Brewers are doing and say, "Hey, man, you're not going to rush me with that crap."
It's possible that opponents will figure it out over time and the impact of fast readiness will decrease. Or maybe a pitcher's psyche is just easily influenced like that and the tactic will continue being effective indefinitely. And then maybe more teams will start teaching this.
I don't know, but I remain at least a little skeptical for now and am interested to see how it plays out over time.