When working with multiple files, you may have one or more window splits
open. You may even be moving back and forth between two in particular --
perhaps if you are iterating on a test and its implementation. You can move
back to the previous window using Ctrl-w Ctrl-w. This is the same as
hitting Ctrl-w w.
Once you've moved back to the previous window, the window you were just in
is now the previous window. That means you can keep hitting Ctrl-w Ctrl-w
to toggle back and forth.
See :h ctrl-w w for the exact command and :h ctrl-w for more on window
commands.