Hmm, unsure. From what I understand it limits the "bloom" effect where the picture gets larger when whites and other bright colors come on the screen. I don't think it actually restricts anything about the white level or contrast or whatever, it just ends the dilemma of either having a distorted picture or a dim screen, since lowering the brightness will reduce distortion overall. If anyone better versed in this can jump in though, it would be appreciated.
EDIT: Looking at Sony's support site, your XBR960 does have a high voltage regulator FYI. The FV310 is the only 480i Trinitron I've found that has one though.