The ZPS2 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 ZPS2 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 ZPS2 system facilitates simpler set-up of the tremolo and easier string changes. Ibanez claims that the ZPS2 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 ZPS2 system can be removed.
The main difference between the ZPS2 and the initial version of the ZPS is that the ZPS2 was used in conjunction with tension adjustment thumbwheel which replaced a hex bolt adjustment system in the first ZR iteration.