It seems to be a common thing among guitar players to have some struggles with well executed legato. Hammer-ons and pull-offs lend themselves to speed so well that guitarists will often rush their notes when it's legato time.
This exercise was given to a guitar student to help them improve their pull-off rhythm/timing. You have one beat of straight picking to get the proper triplet rhythm in your head, then one beat with pull-offs.
Usually, you have to think about pulling off at a slower pace to actually stay at the proper tempo!
Here is the Guitar Pro 6 tab for this lesson:
John Taylor - guitar and bass instructor for Mile High Shred.