They see what a Ruach sees. It is not Ruach Hakdoesh. Different Tzaddikim have different types of Ruach Hakodesh. Mainly, it seems to be an aspect of ????? ???? ??? ??????. And as the Medrash says, whoever advises ??? ???? his advice works out, as it says, ??? ??? ??? ????.
Regular people are also Zoche sometimes to ????? ?? ??? ?????. People relate how they don’t even know why something popped out of their mouth, and how well it played out. In the notes of the Tolner Rebbe, the Gerrer Rebbe remarks how he says what “they ” put into his mouth to say. It is almost that he is as much a bystander as his audience, to his own words.