It was not so much hidden, as it was unknown to me. In the example hg.conf that comes with the mirror download,
kent/src/product/ex.hg.conf
there is this little nugget:
# New browser function as of March 2007. Future browser code will
# have this on by default, and can be turned off with =off
# Initial release of this function requires it to be turned on here.
browser.indelOptions=on