Zimmermann has always had good control. In his 2009 rookie season, he walked just 7.4%. Prior to his call-up that year, his career minor league walk rate was 8.4%. Anything under 8% is very good, and given Zimmermann's raw stuff, it was even more impressive.
But this year he's been truly elite, facing 302 batters so far and unintentionally walking just 15 of them--a minuscule rate of 4.97%.
How good is that? Put it this way: Remember what a hacker Cristian Guzman was? His career walk rate is 4.6%. (BTW--it's weird how Guzman has vanished from the face of the Earth this year. His agent says he's sitting out the season with "family issues." I'm glad I don't have to worry about it as a Nationals fan, but I am kinda curious.)
His strikeouts are also a bit down, from 23.5% to 16.9%, likely the result of an intentional shift in approach to try to throw more strikes and get quicker outs. But that's still a very good rate of missing bats. His velocity is steady, and his breaking balls, especially his slider, are getting better results than ever. Hitters are batting .237 against him this year, compared to .265 in 2009. There's no indication that he's sacrificing anything in terms of stuff.
You're going to prevent runs about as effectively on a per-inning basis with his 2011 rates as he did in 2009. The reason it's a favorable trade-off is that you're also going to be able to pitch deeper in games. He's gone from 5.7 innings per start in his rookie year to 6.2 innings per start this year. And at the same time, his pitches per game started has actually declined from 98 to 93.
These are pretty obvious, ho-hum observations--you walk and strike out fewer batters, you're going to be more efficient and go deeper into games.
But from a developmental perspective, something really exciting is happening. We're seeing a guy who was already pretty damn good develop an even more elite skill set that we didn't know was there. The ability to get hitters out while pounding the strike zone like this is something only a very few pitchers can do consistently. Guys either lack the command or don't have the stuff to throw this many strikes and not get killed.
In the end, Zimmermann's ideal approach might be somewhere in between--a few more walks and a few more Ks. But if he has the ability maintain this degree of extreme command, that means the ceiling for Jordan Zimmermann, already very high, just went even higher.