Watch the Hammer-ons and Pull-offs online guitar lesson by Angus Clark from Hard Rock Survival Guide: Lead
Hammer-ons and pull-offs are another pair of techniques that create a different sort of articulation from the string. Here again, finger strength is paramount, but instead of pitch being so much of an issue as with bending, your sense of rhythm is now at play. When you hammer on and pull off you are creating a rhythm with your left hand instead of with your pick. So practice hammers and pulls with the metronome. In order to develop your ability to hammer on or pull from from any finger to any other finger, look at the exercises in the tab. Typically we are stronger when working between fingers 1 and 2 rather than 3 and 4 or 2 and 3, etc. These exercises will help you even out the volume differential between your picked notes and your hammered or pulled notes.