This sound is produced by simultaneously exhaling into the instrument, and then stopping the air with the tongue. This action produces a percussive effect with a faint pitch, sounding the fundamental of the instrument (Bb, A, Ab, ... E).
Next, some brief improvisations on the open horn:
...and with plunger:
While slap-tonguing is pretty common in reed writing, I've come across only a few examples in brass:
Fractus, for trombone and Super Collider, by Eli Fieldsteel
Ground Round, for trombone and Max/MSP, by Steve Snowden