The ZPS (or Zero Point System) is a tuning stabilizer which was used in conjunction with the ZR tremolo. The ZPS was introduced along with the ZR in 2003.
The ZPS consists of a metal bar attached to a pair of springs. It is installed in the tremolo cavity outboard of the usual tremolo springs. The purpose of the ZPS is to help the tremolo to return consistently to the "zero-point" or neutral position. The zero point is the position at which the tension exerted by the instrument's strings is balanced by the tension of the tremolo springs with no pressure on the tremolo arm and the strings in tune.
The ZPS system facilitates simpler set-up of the tremolo and easier string changes. Ibanez claims that the ZPS prevents the other strings from falling out of tune if a single string should break while playing, but a look at the design and personal experience shows that this claim is overblown. Should the user want the tremolo to function as a fully floating trem with more ability to "flutter" when it is returning to its stable position, the ZPS system can be removed.
This initial version of the ZPS was replaced by the ZPS2 in conjunction with the addition of a tension-adjustment thumbscrew to the ZR tremolo which was introduced for 2005.