There isn't multithreaded rendering QGIS 2.2, it's a feature that will be available in QGIS 2.4.
You can try out QGIS Master (nightly) for testing, and then QGIS 2.4 is scheduled for release on June 20th, 2014.
Whoops, misread your question and from the comments, it looks like multithreaded Processing is something that happens at either the QGIS developer level or the Python Plugin contributor level.
If there is a specific QGIS tool you're using from the menus that's built into QGIS than to get multithreaded processing it would probably need to be coded by a developer for QGIS or even to the root of the tool (GDAL, SAGA, Orfeo, GRASS, R developers, etc...).
If there is a certain plugin than it's most likely coded around Python and the contributor or maintainer of that plugin could see if it's possible to include the
threading modules to enhance its performance.
Either way, submitting a feature request, or seeing if one already exists is usually the best way to get started. It would let you know if someone is already tackling the enhancement or what resources would be needed to kickstart it.