See a “Before” of Just the Panel I’m Working In?

If you want to see what your image looked liked before you made edits in the current panel (for example, if you applied some sharpening in the Detail panel, and you wanted to see a before/after of just the sharpening—not all the changes you made since you first opened the image in Camera Raw), then click on the fourth icon from the left beneath the bottom right of the preview window (shown circled above) or just press Command-Option-P (Ctrl-Alt-P). That will toggle on/off a before/after view of just the edits you made in that one panel. Of course, if you want to see a true before/after of all your changes (no matter where you made them), press the letter P on your keyboard. To see a side-by-side before/after, press the letter Q to toggle through the different before/after layouts (side by side, top and bottom, or split screen).