You know the drill: button 3 (usually right click) to unfold/fold
directories (the ones marked with a '+'.  Same click on a regular file
sends it to the plumber, so default behaviour will be to open it if
it's text.

Button 2 (middle click) on Win or Xplor commands opens a new win (or a
new xplor respectively), by default at the same root directory as
their parent.  However, they make use of chording 2+1: just position
your cursor wherever you want in the tree, then button 2 on Win or
Xplor, hold it, and at the same time press button 1.  Tadaaa!  behold
the chording magic!