With file list active, pressing a letter on the keyboard selects the next file/folder whose name starts with that letter. Pressing that key again selects the next file with such a name. It also used to be possible (konqueror, older versions of dolphin?) to navigate in the backward (i.e. select the previous file/folder whose name starts with a given letter) by pressing the combination Shift+[letter]. I found that feature very useful for quick file searches, especially when the file list is not sorted by name and it would be great to have it back. Without backward navigation on usually presses the key too often such that one needs to continue to do it until one returns to the file searched. Furthermore it is a shortcut combination that similar applications feature as well.
An interesting idea, but I think there's an even easier way to go back to the previous item: just use the appropriate arrow key.
I would not call it "idea" since it isn't new. That feature was present in the past. Typical usecase (where your arrow key won't help): You have a large number of files whose names all start with the same latter. You know that the file you search will be rather be at the end of the list rather than the beginning. It helps tremendously in this situation to be able to sift in the opposite direction, if you give it a thought. Independently, if you are used to using your keyboard, it is a lot quicker to press the shift key in addition to the letter that you already have your finger on, than finding the proper arrow key.
Ah, I see what you mean now.