The pull-off is just the opposite of the hammer-on and results in a lower note. To do a pull-off, fret a string, pick it once and quickly release the fret so that the note below the fretted position sounds without picking the string again. You can pull-off from a fretted note to an open string or from a fretted note to a fretted note. Like the hammer-on you can use the pull-off to play more than two notes by pulling off a second time.
The pull-off is illustrated using tablature in much the same way as the hammer-on. The only difference is that the pull-off has a capital P over the arch, like this.
Using the A string as an example again, the first pair of notes represents a pull-off from the 2nd fret to the open A string. The second set of notes represents a pull-off from the forth fret to the second fret, The third set of notes represents a pull-off that encompasses three notes. Like the hammer-on the pull-off is sounded by a single stroke from the pick.