I think what you’re seeing isn’t Clemens trying to stick his chest out. Rather, he’s keeping his head and shoulder stacked upright. In that picture, Clemens has kept his head and shoulders stacked upright, he has maintained good posture, and he has kept the glove up and in front. His shoulders have rotated around an upright spine and have squared up to the target. What happens next is his torso will flex forward and his arm (still in external rotation in the picture) will snap forward. Of course, Clemens’ timing also appears to be right on. (I know, that’s judging a lot from one still photo.)
These are the things that are important. One thing to be aware of, however, is that it takes flexibility and strength in the low back to have a long stride, get out over the front foot, and still keep the head and shoulders stacked upright. But these are the things you should focus on. I wouldn’t focus on sticking the chest out.