Well a beam splitter as described in the Wikipedia page is moving past my own ability to describe, but the net effect from the double-prism description seems to

Well a beam splitter as described in the Wikipedia page is moving past my own ability to describe, but the net effect from the double-prism description seems to resemble the effect of precisely curving the cable: Some light is able to internally reflect out the splitter while the rest proceeds as normal. But in this case the prism is 'just' a convenient way of getting the desired diagonal shape with materials of different refractiveness, you're not actually breaking light into its component wavelengths as you'd see in NSA's PRISM logo.