In effect, blowing more than one note simultaneously, like an interval - the easiest one to get is on a note like a low C; if you adjust your embouchure slightly while blowing the C, you can introduce an overtone/harmonic that gives you a G above it while still retaining the low C.
I'm certainly no expert, having found these almost by accident when missing low notes, then experimenting to the point where I can control it.
I believe some people can get some more unusual note/overtone combinations, even have the ability to play chords. I imagine this is how John Coltrane got some of his strange tones.
Cheers, Bob