Tremolo Arm, Jazz Guitarists

Instruments without this device are called hard-tail . The term vibrola is also used by some guitar makers to describe their particular tremolo arm designs. The tremolo arm began as a mechanical device for more easily producing the vibrato effects that blues and jazz guitarists had long produced on arch top guitars by manipulating the tailpiece with their picking hand. However, it has also made many sounds possible that could not be produced by the old technique, such as the 1980s-era shred guitar "dive bombing" effect. It may also be used outside it's socket for different effects, as in Whammy Tapping.

Source: Wikipedia > Tremolo Arm