Pull-offs work in reverse to hammer-ons. They are applicable when the second note is lower than the first (going down the neck toward the nut). The example below demonstrates a basic pull-off between two fingers.
Both fingers should be in place as you commence. The index finger is therefore fretting its note before it is actually heard. Instead of simply releasing pressure on the note, the 3rd finger "flicks" in the direction of the floor so as to pluck and release the string at the same time.
Pull-offs style involving notes played by 1st and 3rd fingers.
Instead of using a flicking action, some players actually prefer to simply lift their fingers directly off the string.
Lick 7 is a descending E minor run using the pull-off technique. It is opposite to which that which is used for hammer-ons ascending.