Pull-offs begin by
placing both fingers on the notes to be sounded.
Instead of bringing your finger
down onto the string to sound a new note, you lift your finger off
of the string to sound a new note.
Note that simply
lifting your finger off of the string won't do it. The secret to
executing pull-offs correctly is to actually "tug" or "pull" the
string slightly with your finger before releasing it to sound
the new note.