the frist level of yafray integration into blender was to output an XML (extended markup language - google for that for further information) file that yafray undestands. in general, it’s a human-readable text file that describes the scene - objects, light, camera position, background etc. yafray reads that file and renders the image. now, yafray supports more/different attributes for various settings than blender supports. so, you could edit the XML file and tweak it to switch on/off all those attributes you need, e.g. volumetric lights.
this is, at least for large scenes, a pain in the as#. this is the part where yablex kicks in. it is a python-script running in blender which lets you tweak loads of settings of the scene right out of blender. you simply select an object, edit it’s (yafray-relevant) settings, hit apply. when finished, you hit “export” to write the XML file which is then ready to be passed to yafray for the final render.
hope that gave you a general overview - for detaisl on which settings you#ll need to edit pls check out the yafray documentation or the documentation of the yablex script.