String bending is a guitar technique where fretted strings are displaced by application of a force by the fretting fingers in a direction perpendicular to their vibrating length. This has the net effect of increasing the pitch of a note. String-bending allows exploration of microtonality and can be used to give a distinctive vocal articulation to lead guitar passages.
String bending is executed by fretting a note on the guitar fretboard, and then applying a force perpendicular to the length of the fretboard with the fretting hand, displacing the string from its resting vibrating position. This yields a continuous increase in pitch, which can be manipulated by a skillful player to give a singing-like quality to a musical passage. The displacement of the string can be pushed "up" or pulled "down". Bending is an important component in the style of several renowned players, such as Eric Clapton  who uses copious amounts of string bending to articulate blues licks. Jimi Hendrix used the string-bending intro for "Foxy Lady".
Factors influencing string bending
There are numerous mechanical and acoustic properties which heavily influence the resultant pitch from a string bend. Analysis of the physics of string bending  suggests that the resultant pitch of a string bend is given by
where L is the length of the vibrating element, T is the tension of the string prior to bending, is the bend angle, E is the Young's Modulus of the string material, A is the string cross sectional area and is the linear density of the string material.
Thus, the pitch is not only dependent on the bend angle, but on material properties of the string such as Young's modulus; this may be interpreted as a measure of the stiffness of the string. The force required to bend a string to a given angle is given by
It is important to note that the resultant pitch from string bending is not linearly correlated with the bending angle, and so a player's experience and intuition is important for accurate pitch modulation.