Watch the Thumb-index Roll online guitar lesson by TrueFire from Play Acoustic Guitar 8: Advanced Techniques
Thumb-index Roll - Technique #14 is a video guitar lesson presented by Muriel Anderson and is sourced from 50 Right-Hand Techniques You Must Know.
Draw the thumb down across the strings until you get to the first string, then pick it up with your index finger and pull the arm back in the opposite direction.
This creates a rolling sound that you hear often in Flamenco music. Try doing it not too fast so that you can hear all of the strings.