Tremolo Arm, Machine Screws

The tailpiece consists of a solid block of metal, mounted behind the tremolo plate and secured to it by three machine screws, and passing right through the guitar body. In a chamber routed into the back of the guitar are up to five (normally three) long coil springs which connect to the back of the tailpiece block, and whose tension balances that of the strings. The tremolo arm also passes through the tremolo plate and tailpiece block, providing direct and rigid connection. Ignoring the bridge adjustments, this mechanism has only two moving parts, one of them the arm itself, the same as the Bigsby. But unlike the Bigsby, the synchronized tremolo moves the bridge as well as the tailpiece, varying both the length and tension of the strings.

Source: Wikipedia > Tremolo Arm