You've made me wonder how related the yodel is to the overtone.  I think the overtone is the simulatneous production of different pitched tones, while the yodel is a sudden break up the register.

The country and western yodel, Dolly Parton style (2:33):

(I love this recording, great space, sounds really fresh.  Really lovely leap at 2:05, no pics just the music.)

(btw, as I understand it overtones are created by most instruments--including the voice.  The flute (metallic) doesn't create overtones which gives it that piercing sound and makes it hard to...er...well, apparently Mozart hated composing for it.)

Don't fight forces, use them R. Buckminster Fuller.