Bending strings is a guitar technique primarily used in playing single note riffs and in lead guitar situations. Effective use of string bends can emote a "vocal" quality from the guitar. Although it's a technique used mostly by lead guitar players, even three chord folk guitarists will probably be required to use string bends from time to time. Be aware, however, that bending strings on acoustic guitar is a much more challenging undertaking than it is on electric.The classic string bending technique involves fretting a note using the ring (third) finger, with support from second and first fingers, and bending the string upwards (towards the sky) until it reaches the desired pitch. The overwhelming majority of string bends take place on the top three strings (G, B and E) of the guitar, as they are the lightest gauge and easiest to bend. We'll apply these principles in the exercises outlined below.
Basic Bending Technique
Our goal for this basic bend is to play the note on the 10th fret (the note A) of the second string, bend the note up a half-step so it sounds like the note on the 11th fret (the note Bb), and then return the string to it's unbent position (A). To prepare your ear for what this should sound like, play the 10th fret of the second string, and then slide your finger up to the 11th fret, and play that. The note on the 11th fret is your "target pitch" - the proper pitch of the note you're aiming for in your bend.
Start by fretting the note on the 10th fret of the second string using your third finger. Although they aren't responsible for playing any notes, your second finger should rest behind your third finger on the ninth fret, and your first finger on the eighth fret. Bending the strings far enough to get the pitch to change takes a good deal of effort - you'll want all three fingers to aid in bending.
Now that your fingers are in proper position, play the second string, and exert force in an upwards motion (towards the sky), while still keeping enough pressure on the string to keep it in contact with the frets. Make a conscious effort to use all three fingers in your bend, not just the third finger. When you've bent the string enough to reach the desired pitch, return the string to it's original position.
Chances are, when you first attempt this, you won't get the pitch to change much. This will be especially true if you try bending on an acoustic guitar - they are much harder to bend strings on. Be extremely patient... chances are you haven't used these muscles before, and they will take time to strengthen. Keep practicing, and you'll get the hang of it soon.