The gentleman I was referring to was Paul Chapuis, former solo trombonist with the Basel Radio Orchestra who then took over bass bone and then retired when the radio orchetra was merged with the symphony orchestra. We taught at the same school and my wife had trombone lessons with him while I was still teaching trumpet.
I don't speak French, but if I'm not mistaken the "tiu" you mention is done in a manner, that the tip of the tongue is anchored at the base of the lower teeth while the forward middle part of teh tongue arches against the upper palate to make the sound.
When I do my "seed spitting" for lower valve and pedal notes, my jaw is dropped slightly forward and after the tip of the tongue strikes the lips, the tip regress quickly and lays flat on the bottom of my mouth.
Oh, and greetings from snowy Lupsingen just outside of Liestal in BL.
Your "tiu" seems to be what I call by "dorsal tounging" common in Sweden. The tounging between the teeth is common for tubaists French horninsts and trombonists in very lowe range, usually the jaw drops and move slightly forward. But the lips are still in contact.
Some players canīt do that, maybe they drop the jaw to much? Some players do have a sympatethic conection between the jaw and lower lip, when they lower the jaw the lower lip follow down and the apperture get to large. It can be cured sometimes at least.
Note " tounging betwen the teeth donīt nessecary means a bigger aperture at rest"
When you are playing a high tone has two appertures that the lips are moving between, close and a small.
When you are playing a low tone the two appertures are closed and big. On a very low tone the big apperture is very big, bur the small is still closed.
That said, if your playing is fine, donīt change anything!