Are you talking about the mouthpiece he is blowing into?.
If so, it is a breath controller...looks like a Yamaha model...I have the other version, used on my CS-01, which looks like a whistle. Many Yamaha synthesizers (DX-7, DX-5, CS-01) and controllers (KX-88, KX-76 KX-1, KX-5) had this feature...although I believe he (David Sancious) is playing a Peavey Controller, which had similar capability
The breath controller he's using is a later version, and can be worn like a headset.
Can be used to control volume, filter, etc.
Very expressive indeed.
Ian
PS...you used to be able to buy a unit by "MIDI Solutions" that let you plug in a breath controller, and midi the device to your synth...also, I think Kurzweil may have had a breath controller input on some models.
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.