Palm muting

The palm mute is a playing technique for guitar and bass guitar, executed by placing the side of the picking hand below the little finger across the strings to be plucked, very close to the bridge, and then plucking the strings while the damping is in effect. This produces a muted sound. The name is a slight misnomer, as the muting is performed by the side of the hand, not the palm.

Palm muting is a standard technique used in classical guitar performance (under the name of pizzicato, as it creates a sound similar to that of a bowed string instrument when finger picked, despite a very different construction from that of a guitar) and by electric guitarists who play with a pick. Palm muting is so widely used as to be idiomatic in heavy metal, and particularly in thrash, speed and death metal, but it is often found in any style of music that features electric guitars with distortion in the signal's preamplification stage. It is responsible for the characteristic "chugging" and "crunch" sound of distorted guitar music. In metal, palm muting makes chords and notes sound shorter and more defined. Palm muting can also be used in conjunction with a wah pedal to produce the distinctive scratching sound often heard in disco music.

Palm-muting is also used by electric bassists in order to obtain a warm, "thumpy" tone that is sometimes similar to that of a finger picked double bass (as noted above). The strings may be plucked with the thumb, or with a pick which gives a more percussive tone.

There are many ways to perform palm muting, but, generally the following are recognized:

Palm muting is a basis for many other techniques, especially those specific to electric guitars, such as sweep picking or alternate picking.

In guitar tablature, palm mutes are rendered with a "P.M." or "PM", and a dashed or dotted line for the duration of the phrase to be muted. If the pitches of the muted notes are discernible, the fret numbers are given accordingly, otherwise they are represented with an X in lieu of a tab number. (If an X appears in lieu of a tab number but there is no P.M. directive, this usually means to mute the string using the fretting hand, not the picking hand.)

...
Wikipedia