Using tags and ctrl-] is a quick way to jump from the use of a keyword to
the declaration or definition whether in the same file or another file.
Sometimes what you really want is that tag definition opened up in a
(horizontal) split window. That way you can see the definition without
losing context. This can be accomplished with ctrl-w ].
The Vim help file gives the following definition of this command:
Use identifier under cursor as a tag and jump to it in the new upper window.
See :h CTRL-W_] for more details.
h/t Dorian Karter