Thiocyanate works like sulfite on steroids, i.e. it is a powerful solvent, very similar to thiosulfate. Added to a developer it amplifies physical development and thereby generally speeds up development, but also eventually creates fog - at first regular fog, and at some point dichroic fog. The more fog it creates, the weaker your shadow regions will be after reversal. During second developer stage fog is a non issue, as all the silver halide left is meant to be developed. The 'it clears up highlights' feature comes from the contrast increase it creates.