[XeTeX] XeTeX, font protrusion, and the microtype package

[Philip TAYLOR]

Could anyone please tell me the XeTeX primitives into which
the Microtype package hooks in order to enable font protrusion ?
The output of "TeXdoc XeTeX" makes no reference to any such
primitives.  I would like to be able to make use of whatever
microtypographic features XeTeX offers, but of course being
a non-LaTeX user pre-packaged solutions such as the Microtype
package are of no direct use to me.

Philip Taylor