On the list of things most often done wrong on guitar, the Pull-off holds a respectable place. In fact, I remember I was playing for years before a teacher pointed out to me that I was doing them incorrectly, resulting in either distorted rhythm, or missing notes.
A pull off means playing a note and then pulling the string downward toward the floor with the left hand finger used to fret the note, and then releasing it so a second note is sounded by just the force of the release. You can pull to an open string, or (more common) to another note already prepared by a left hand finger.
If going to an already prepared note, keep these things in mind:
1) The holding finger MUST hold the string firmly, straight to the fret, and not allow the string to move at the fret where it is held by the "receiving finger" during the pull off. It often does NOT hold the string firmly, and should be checked in a mirror.
2) Very often a player will not really pull the finger, but will just lift it straight off the string. This does not keep the string in vibration, and the second note does not come out, or is very weak, especially at fast speeds.
Depending on the speed of the music, and other considerations, both fingers may be placed at the same time, the first is plucked, then pulls off, keeping the string vibrating so that the second note is heard even though it is not plucked by the right hand.
So when you do pull-offs make sure of this: the pulling finger pulls the string, down a bit toward the floor, while the holding finger holds the string, pinned down as it were to the fret board, and not moving along with the pulling finger. Then both notes will be clear.
A note of warning. This action requires strength and control, which must be developed carefully, or harmful tension will be locked in to the muscles during practice. As much as you know and understand about "The Principles of Correct Practice," use them in practicing Pull-offs.