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It is this vibration and movement which causes the tickling sensation for the patient, who is of course numbed to the pain of the cannula inside them by the infiltration of the anaesthetic. The nutational vibration of the cannula is also said to interfere with the brain's pain receptors in the same way that scratching relieves an itch.
The technology used is a 3D acoustic energy which is emitted as a low, infrasonic frequency (10Hz), said to be safer than ultrasonic waves as this means that the cannula remains cool during the procedure so poses less risk to surrounding tissue.