The milky fluid you can see during prostate milking is nothing more than the prostate fluid stored in the gland.  This fluid is static in the gland waiting to be released at the time of ejaculation, which occurs because the gland has a muscular layer that contracts during ejaculation and releases the fluid to the outside.  However, prostate milking stimulates the same muscle layer. It causes a release of the contents of the prostate in the absence of a full orgasm.