Sometimes thumbnails are just too small to let you see all the details in an image. If you don't have a second display and you would like to apply a group of keywords while looking at the Loupe view of an image here's an easy way.
Begin by setting up your Keyword Shortcut. You find this under the Metadata menu in the Library module. You can also use the keyboard shortcut Command-Option-Shift-K on a Mac or Ctrl-Alt-Shift-K on a PC. This will present you with a deceptively simple dialog into which you can enter any number of keywords separated by commas.
Once you have the keywords you want entered click the Set button.
Select the first image in Grid view and switch to Loupe view (press E). If you want to apply the keywords press Shift-K. To move to the next image press the right arrow key. If the new image gets the keywords press Shift-K or skip it by pressing the right arrow key. Work your way through... Shift-K, right arrow, right arrow, Shift-K, right arrow, and so on.
If you mistakenly apply the keywords just press Shift-K a second time to remove them.
If you look at your Keyword list and notice a small + after a keyword, that's a signal that the keyword is currently part of the Keyword Shortcut.