Looking at the sources isn't so bad
Here's a part of ext/graphics/graphicsmodule.cpp file:
This function is used for example in blit() method to convert a PyS60 image object to CFbsBitmap object (which as I believe is a Symbian bitmap).
static CFbsBitmap *
CFbsBitmap *bitmap=(CFbsBitmap *)PyCObject_AsVoidPtr(capi_object);
This code is not complete, for example the PyCAPI_GetCAPI function is found in the same file but I won't copy it here. Like I said, take a look at the sources.
Of course if someone can explain it without looking at the sources, it would be great.