Pull-offs are often challenging for most people, as they tend to just throw their fingers down when moving to lower notes, or just letting go of the string instead of actually making a proper pull-off motion.
This exercise always follows the pattern of pick once, pull-off twice. It is important to have your fingers down on the notes to be played BEFORE you begin the pull-off motion in order to create the best sound possible.
Here is the Guitar Pro 6 tab for this lesson:
John Taylor - guitar and bass instructor for Mile High Shred.