The Ibanez Handrest was designed by Steve Vai and released in 1987 with some early model JEM777LNG, DY and SK guitars. Due to a potential patent infringement the "handrest" was not widely distributed and thus came with no rhyme or reason with early 777 guitars. The handrest has never been sold to the public through dealers or from Ibanez.
The purpose of the Handrest was to allow Steve Vai to rest his wrist on the tremolo without causing thet floating bridge to pull sharp. The handrest slips over the tremolo as shown in the pictures below. The fine tuning screws by the tuners allows the "height" to be set so that the handrest does not interfere with the bridge or cause it to pull sharp. Once adjusted you can put pressure on the handrest and the tremolo will not go sharp.
Handrest - resting position
Handrest - pivoting shown
Handrest - full pivot
Top view removed from guitar. The screws on top attach to
the tremolo post. The screws on bottom allow for adjustment
of the handrest for clearance from the tremolo.
Underside view. The hinges on top are the point of attachment.
The tremolo arm fits thru the oval cutout.