:help CTRL-^
or
:help ^^
works for me.
It's not just vim, either. The ^^ toggle-buffer command is is a standard vi feature going all the way back to ancient times.
In vim you also get the ^I and ^O commands which take you along a stack of previous positions, and they can jump from one buffer to another if you reach a point in the history where you had switched buffers.
And then there's :n[ext] :p[rev] and :rew[ind] which move you along the list of files that were named in the original command line. Those are also classical vi (ex) commands. vim adds some commands for altering that list of filenames too. See :help arglist