These trills, called tahrir,
are made by rapidly closing the Glottis, effectively breaking the notes(the effect is
reminiscent of Swiss yodeling).
When these muscles vibrate, they open and close the Glottis(the opening between the vocal cords),
allowing for sound to be created on inspiration and expiration.
While breathing air through the gills,
a tadpole will take in water and close its Glottis at the same time,
allowing the water to be forced out through the gills and not into the lungs.