Like the human voice, the sound of the trombone is very rich with overtones. And, similar to overtone-singing, its overtones (or formants) can be highlighted depending changes in the oral cavity of the performer.
Some find it helpful to think of vowel changes to bring out these effects. Here I play using these discrete vowels "o, a, e, u, ü, ..., a, o":
However, I have found I can produce more defined formants if I focus on the pitches themselves, rather than discrete vowels. Here I oscillate from the fundamental to my upper limit:
Here is an brief improv, that explores these techniques a little further:
The talented Isreali composer (and accomplished overtone singer) Naftali Schindler has written me a piece called Two-Eyed Elk that explores some of these techniques. Here is a brief excerpt:
Finally, speaking of music at the convergence of overtone singing and the trombone, check out this video of the brilliant Roswell Rudd and his BLUE MONGOL project: