It does multiply or add the brightness, pretty much like when smoke passes some sunbeams eg. from a window. the smoke reflects the sunlight and appears to be bright. The lightbeams are visible by their own as well since there is dust or whatever in the air. But both, lightbeams by their own or smoke in the shadow are only half as bright as the smoke in the sunbeams. For the sunbeams you only need to set the EntityFX to Fullbright. The texture should use an Alphachannel to allow to fade it out smoothly.
There might be many ways to achieve this effect and this method I just described might have pros AND cons.
There might be many ways to achieve this effect and this method I just described might have pros AND cons.