The palm mute (known as pizzicato in Classical guitar terminology) is performed with the picking hand. While picking, the side of the palm is lightly pressed against the strings to mute the notes. You’ll see it labeled in tablature with “P.M.”.
You can vary the sound of a palm mute by adjusting the applied pressure. For example, light muting can be done with a light touch and heavy muting with a heavy touch. Also, palm muting near the bridge will produce a lighter muting, whereas a heavier muting can be produced by palm muting further from the bridge.
The name is a slight misnomer, as the muting is usually performed by the side or heel of the hand.
While rare in classical guitar techniques, palm muting is now a standard technique among electric guitarists who play with a pick. Palm-muting is so widely used as to be idiomatic in discrete music, heavy metal, and particularly 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” sound of distorted guitar music. 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 double bass. The strings may be plucked with the thumb, or with a pick that gives a more percussive tone.