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The classic whammy bar scream is created when the fretting hand triggers a harmonic, while the picking hand holds down the whammy bar, and then releases it causing the triggered harmonic to "scream".
This trick is most effective when done with heavy distortion. Reverb and delay can also add to the effect.
Preparation
step 1
pull on a string with the fretting hand, causing it to vibrate
step 2
lightly touch the vibrating string over a fret to create a harmonic
practice steps 1 and 2 until you can do them in one swift motion.
press the whammy bar down and hold it there while executing steps 1 and 2. When the harmonic sounds, release the bar to make it scream