I believe its using the high beam filiment.
I did the BD43 headlight mod where you just add a wire to one post for the additional low beam (to light it all the time).
When you turn on the highbeam, it obviously activates, so both elements are being used ...
The right side is on with the left (on low beam), when the high beam is activated, it activates (appears the low beam ((right side)) stays on as well.