I know I can use the up and down arrows to navigate through the matches and the return key (or F4) for jumping to the current match in the main editor.
The documentation mentions Cmd-down arrow, but for me (Mac OS X keymap), it seems to do nothing.
When processing a lot of matches, sometimes I want to edit each match, and it is a little tedious to use the mouse to click into the source preview pane. I also don't love hitting the return key to go to the result in the main editor, especially if I leave some instances alone (because then I have to skip past those each time I go back to "Find In Path").
I would ideally like to be able to use a keyboard shortcut to do this efficiently. I could imagine keyboard shortcuts for navigating through the matches *in the source preview pane*, or I could imagine a shortcut for toggling focus between the preview pane and the search term box. The documentation makes it sound as if the Cmd-arrow shortcut might be intended to do the latter, but I'm not sure, and since it doesn't work for me, I can't tell.