How do indent/unindent by a keyboard shortcut?

What is the keyboard shortcut to indent/unindent parts of a list?
I used it accidentally earlier but can’t seem to find it again.

Also, I was not able to find anything by searching for it. I know it has to be somewhere, but whats the supposed way for a user to find out how things like this work?

Ok, so by trying to delete a word via

Ctrl + Backspace

I found out that this was my accidentally triggered unindent option. However it does not work quite as I hoped it would. In the following case:

  • In Section 2 we do X
    • Also do Y
    • What we do then will shock you!
    • We complete it by doing Z

Now I only want to unindent the 3rd and 4th item (“What we will do…” and “We complete it by doing”).

Is there a way to do so?

I think you misunderstood what happened. Cltr+Backspace eliminate the innermost environment around the cursor. In your case the second-level itemize environment, making seems that you have “unindented” the items all at once. There is no easy builtin way to change the itemization level of a specific item. See the discussion here, for example: Itemized list editing is a bit counterintuitive? .

Note also that removing the innermost environment should be done with care, as sometimes the innermost environment is not really shown in the page, e.g. when doing beamer slides and you might accidentally erase a whole lot of document structure.