what exactly is an attunement?

..using NEW terminology, what exactly is it? On most reiki sites, it says something like 'puts you in tune with the healing light' etc. but whats the NEW take on it?

also, I was wondering how beneficial it would be to provide a very negative person with an attunement as a means of making them more positive long term?

Well, an attunement (in Reiki terminology) is when a person is surrounded with a specific energy (or energy vibrating at a specific frequency, or with modulated with a specific information), and the practicioner is then entrained to this frequency (once again, think vibration- causing your vibration to entrain to it, or another way to describe it would be that your energy becomes encoded with the energetic information (in this case think modulation), making your energy body carry this modulated information in it.
If this is not clear, think of a microwave- how it works is that it gets the molecules of matter to vibrate at the same rate as the microwave (which by it's name you can know it's at a high frequency)- it entrains food to vibrate at the speed of the microwaves which causes them to get hot. Of course, in terms of attunement, it's your energy body that's made to vibrate at the specific rate of the intended vibration.
However, the use of symbols in Reiki makes me think that attunement is not so much entrainment as modulation- now think of a radio- where a signal that is too 'slow' to travel long distances is encoded into another signal that is much higher, carrying it long distances, to then be decoded by a receiver, which turns it into audible sound or video for your enjoyment. If I were to guess I'd guess Reiki is more like modulation, and you become attuned to the specific code (Reiki symbol) that is encoded in your energy body, acquiring whichever code (symbol) is used in the ceremony.
Of course, this is me making some sort of sense of how I think these things work. I'm not knowledgeable in any of these specific areas.

I don't know how this can be 'translated' to tactile visualization, because NEW is not a specific type of energy, but a way to get feedback as it flows throughout your body.