Watch the Pull-Off online guitar lesson by Chris Buono from 60 Electric Guitar Techniques You MUST Know
What could be deemed as the opposite of hammer-ons, pull-offs are a descending legato technique where you pick a fretted note and then that same finger down towards the floor thus activating another fretted note or an open string. The very nature of this technique makes pull-offs sound a bit more pointed in attack. To ensure the phrasing is strong, I strongly suggest you make sure you're literally pulling off towards the floor and not up off the neck. The latter will garner a weak result setting up your lick or riff to fail.
Looking for inspiration? Usually a player who possesses an inspiring hammer-on technique also plays pull-offs with command. One defining moment in my developing years was Steve Vai's blazing into solo to "Shy Boy". THAT'S some pull-offs!Guitar Gym: Hammer-Ons & Pull-Offs
is just as much pull-offs as it is hammer-ons. Dig in!