Been listening to James Newton Howard's score to Nanny McPhee again where I picked up parts of this device. Specifically it was the glock/violins/flutes Ostinato with the melody below it all in the celli that caught my ear in the original.
I've added the chords above the celesta staff. As the title of the post says it's triads in a tritone relationship. Well, technically it's not all triads since I use a Cmaj7, but the sound really comes from the movement to a triad built from the tritone.
Starts out with the melody in the low register and then works it's way upwards through the different instruments/registers. Since the melody starts out in the low register I introduce the bass little by little as the melody climbs higher.
Once it reaches the highest variation of the melody in the flute it's in a very strong and projecting register. Tried to really phrase the melody in a more idiomatic manner for the flute, instead of simply repeating the horn melody two octaves higher.
In red we have an element called a countermelody. The objective here is to have a secondary line that's melodic enough to be perceived as a melody, while at the same time not upstaging the primary melody. The easiest way to do this is to have it in a separate register from the main/primary melody, and also to rest when the other is active and vice versa. It's a very common element usually found on the re-statement(s) of the main melody, to add interest and energy to the piece.