It's just a euphemism for something that simply sounds more authentic. Being more authentic gets you more involved with the sound and gets you to toe tapping.
I think the mystery behind it that we're looking at it with a modern perspective that can't understand why something so mid range centric as the Naim/Linn sound back in good old days can be called authentic when modern systems can beat them at that game.
Remember that the midrange accounts for about 80% and once they got that down convincingly, PRaT became a thing.
Practically all modern systems have the ability to sound pretty authentic but we still find PRaT to be elusive and that all boils down to the quality of the recording: how well it was done. Great sound in, great sound out.
All the best,
Nonoise