Go Back To The Previous Window

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.