Posted on YouTube by HarmonicaTool:
Harmonica reeds in stroboscopy: See how the reeds of a harmonica vibrate.
Basically there is a 'blow reed' that vibrates while blowing and a 'draw reed' vibrating whilst you draw. Advanced playing techniques -called 'bending' and 'overblowing' allow reeds to switch functions. When the harmonica player is 'bending the tone' both reed do vibrate at the same time. It is used to produce a lower voice and has some special sound characteristics that made the harmonica so usefull for the blues (it does not sound good here because I overdid it for demonstration purposes). In overblowing the harmonica player switches the reeds functions. She/he makes the draw reed vibrate whilst blowing. This produces a higher sound (here again, not beautifull to the ear but impressive to see).
Thanks to Prof. Dr. Neuschaefer-Rube from the Universitätsklinikum Aachen (RWTH Aachen University) for letting me use the stroboscope which is usually used to examine patient's vocal folds vibration.