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http://www.guitarcontrol.com/youtube?v=3_WbWMJ1JVM

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Hammer ons on pull offs are what flashy guitar playing is made out of! A lot of the super fast guitar playing that you hear is usually a combination. Especially blues, and all genres of rock. A lot of speed playing in metal is straight picking, but you can bet they
have their hammer on and pull off on the guitar too!
The key is to only pick your string once! Watch AC/DC's music video for "Thunderstruck"- on the intro, Angus has one hand up, but the other is ripping that iconic guitar intro...
how's he doing it? A mixture of hammer ons and pull offs on guitar.
So by picking an open string you can slam your finger into a fret making a "hammer on".
By fretting a note and peeling your finger off the string so hard that it rings open that's a "pull off".
That's how I would suggest trying them to start but that's not to say that Ho's and Po's can't be used by combining 2 fretted notes as well.

http://www.guitarcontrol.com/youtube/?v=3_WbWMJ1JVM&gl=582827528