The Whites control is new, and works very much like Blacks, only on the other end of the histogram. Pushing Whites to the right doesn't touch the darkest tones in your image, but expands the rest of the range (with the effect weighted in the highlights), clipping the Whites. The summary for Whites is that moving it to the right increases contrast everywhere, while lightening the overall image slightly, and clipping the very lightest tones to pure white. For both Blacks and Whites, the combination of a slight expansion in the midtones along with the clipping gives you very nice control over your image contrast.
Highlights And ShadowsWith the new Tone Controls all starting in the middle at 0, you probably noticed by now that pushing any one of them to the right makes that respective part of the image brighter and pulling any one of them to the left makes something darker. So, in that sense, we now have more symmetry in the controls. But that still leaves the question of how Highlights and Shadows are different from Whites and Blacks. If you just push them around a little bit on a few images, at first you might think they're doing exactly the same thing.
A closer look reveals that these new controls are completely different. Highlights and Shadows do move similar portions of your histogram around—relative to Whites and Blacks—but they also have (in the words of the Adobe engineers) "edge awareness." This is very much like the Clarity control.
As an example, let's look at Fig. 3, the silhouette of the man in the walkway. The exposure is about where it should be given the circumstances, but the tones in the scene exceed that of the camera's dynamic range. Luckily, there's still detail in both the highlights and the shadows that I would like to recover.
Pushing the Shadows control up to +85 (Fig. 4a) not only opens up the shadows nicely, but also brings better definition into the dark areas—a result of the edge awareness (Fig. 4). If that somewhat dramatic move on the shadows leaves you feeling that the photo is now a little flat, this is where you can use Shadows with Blacks together, only moving them in opposite directions. Pulling Blacks down to -25 (Fig. 5a) brings back the contrast, leaving the photo with a more natural, photographic look (Fig. 5).
Finally, to deal with the burned-out walkway, simply pulling Highlights back to -50 (Fig. 6a) brings in a nice amount of detail from tones that were all very nearly clipping in the original capture. Detail is enhanced by the new edge awareness (Fig. 6).
By now I think you can probably see how many variations there must be. So, let's summarize. In general, I most frequently use positive Shadows settings (moving to the right) and negative Highlights settings (to the left) to recover blocked-up shadows or burned-out highlights, respectively. And for Blacks and Whites, it's the opposite. I generally use negative Blacks settings or positive settings on the Whites control to fine-tune clipping points and to adjust contrast. In general, those cases cover 95% of my needs.
Having said all that, what about the other side of the coin? Would there ever be a time when you would use negative Shadows settings or positive Highlights values? And likewise, would you ever need to use positive Black settings or negative Whites values to moderate clipping?