[Proj] Difference between +nadgrids=@null and +towgs84=0,0,0

[Aaron Friesen]

Everyone,

>From the FAQ "Changing Ellipsoid / Why can't I convert from WGS84
to Google Earch /Virtual Globe Mercator?" it says to add

+nadgrids=@null to "trick proj.4 into applying no change to
the lat/long values when going to WGS84".

How is that different than +towgs84=0,0,0?

Thanks,

Aaron