This is a bit of a larger question -- how does anyone who is not a Ringwraith know what a Ringwraith experiences on that side while wearing their rings? Can we take for granted that Isildur put on the Ring more that one time while trying to escape the Orcs at the Gladden Fields. We have to think it would, to know that it made him invisible and thus would allow him to escape the Orcs. If so, he might have been able to share what the experience was for others on this side, in a similar way that he shared what the fiery letters on the Ring were. Not to say that this is the same experience as the Ringwraiths who have completely faded, but at least it might be intel that could be extrapolated from. Or could it be that the Nine, when they were not yet Ringwraiths, were relating their experiences over there as they slowly went from men to wraiths? Again, the earliest levels of ring usage might not be the same as when they are full-blown wraiths, but it could be something that the Wise could work with in figuring out what it would be like.