ttfautohint uses a noop hinter for glyphs in the private use area:
http://www.freetype.org/ttfautohint/doc/ttfautohint.html#scripts

unless you specify the fallback option:
http://www.freetype.org/ttfautohint/doc/ttfautohint.html#fallback-script

Before this change if you opened the generated font in FontForge
and looked at a glyph then Hint > Edit Instructions there would be
no instructions, after adding this flag, the glyph will have instructions.
Merge pull request #124 from yapplabs/fix-hinting

Fix autohinting.