No, it does automatic differentiation.

The overall idea is that if your inputs know their derivatives ( for
instance, when raytracing a primitive, computing P, dP/dx and dP/dy is
easy )
and your shading library know how to differentiate all its operators,
you are able to maintain derivatives quantities while shading.

Look at http://jgt.akpeters.com/papers/Piponi04/ for a complete
reference.


Hopes this helps

Henri