The documentation is a bit cryptic for those two shortcuts.
Looking at the code it seems that they are only for the dialogs relating to setting actions, conditions and their parameters. With those you can move back and forth the different dialogs that are presented.
The back shortcut also works in the expressions dictionary.
An additional extra caveat which I just found out, which is not documented, is that the shortcuts do not work when an input element has focus, and in the case of most of those dialogs, the search input will automatically have focus, which means the shortcuts won't work!
For that there are variants, Alt + Shift + B and Alt + Shift + N, for back and next respectively, those will have the intended effect regardless of an input element having focus.
The first step would be to document those hidden shortcuts... but I am not really sure why they are needed in the first place.