Firstly, the frequency of vibration is very low, and the folds are in fact closed for more time than they are open.
Secondly, the folds are bunched up and thick, rather than being tensed tightly along their length. This allows slow vibration at a low airflow rate. Due to the required low airflow, it’s not really possible to increase the volume of creaky voice, as you can for normal phonation.
Now play the movie of creaky [i]. Notice how small the length of the folds is. You should also get a better sense of the movement of the folds because the frequency of their vibration is so much lower than normal voicing.
The speed of creaky fold vibration is so slow and irregular that it does not work well with a stroboscope.
To produce creaky voice yourself, try saying the vowel [i], imitating a slowly opening creaky door in a scary movie. It helps to keep the airflow at a very low rate. Once you’ve mastered this – try increasing the volume – you should find this difficult beyond a slight increase.
Some languages have creaky vowels and consonants that contrast with normal voicing. Sometimes these sounds are also called ‘laryngealised’ sounds.