Translucency is handled as a special case of transparency, i.e. to use translucency there must first exist some level of transparency, and the refr_trans_w parameter decides how much of this is used as transparency and how much is translucency:
If refr_trans_w is 0.0, all of the transparency is used for transparency.
If refr_trans_w is 0.5, half of the transparency is used for transparency and half is used for translucency.
If refr_trans_w is 1.0, all of the transparency is used for translucency and there is no actual transparency.