| "Mwah" is caused by the string vibrating against (buzzing if you like) the wood of the fingerboard. The string to board angle has to be exactly right for it to occur, although experienced fretless players can induce it often where a novice cannot. The effect can be controlled, modified, and even extended by the fingertip control, and the use of vibrato, much in the way a cellist uses vibrato.
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Ampeg Portaflex Club #249.
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