What causes vocal cords to open?

Your vocal cords open when you breathe and close tightly when you swallow. When you speak or sing, your vocal cords close and your lungs send air through them, causing them to vibrate and make sound. Vocal cords play a critical role in helping us talk, breathe and swallow.

What causes the vocal cords to start opening or closing?

The folds vibrate when they are closed to obstruct the airflow through the glottis, the space between the folds: they are forced open by increased air pressure in the lungs, and closed again as the air rushes past the folds, lowering the pressure (Bernoulli's principle).