It is done when the Index finger is bent while doing the Pataka Mudra. Here the index finger is bent along with the thumb, while the other fingers are held straight.
Lets look at the viniyoga shloka for Aarala:
Vishadhya-amruta paneshu prachanda pavanepicha.
It is used to denote the following.
- Drinking poison , nectar etc
- Violent wind