It doesn't actually convert a RHD lamp into a LHD lamp as such. It's a simple masking device that removes the so-called kick-up on the left side of the dipped beam. This is a standard RHD dipped beam pattern with a kick-up.
With the beam mask in place, you get a flat topped beam.
This avoids oncoming drivers being dazzled when driving on the right. It's a temporary beam adjustment for Continental driving over short periods/holidays. LHD cars have to be fitted with LHD lamps, though I believe flat topped dip beams are OK for the UK MOT test.
Without the kick-up, the nearside road/verge/signage is less illuminated than usual.