I think the whole problem of teachers and their different way to express things comes down to: what is sati-sampajanna? How does one recognise one is indeed experiencing sati-sampajanna? Cause it is not easy to discern and recognize this specific kind of awareness. But it is sati-sampajanna we are required to develop so I think it is an important question and important answer.
There are a multitude of awarenesses. One is always kind of aware when awake for example. One is aware during dream and differently aware during lucid dream. The sati-sampajanna awareness is different from all these, though.
There is a meditative experience that is rather common so I will use it as a starting point because I think most of you will recognise it from own experience:
After two weeks of practice I was able to maintain concentration on the object for a minute or more at a time. This partial success in concentrating brought with it certain pleasant experiences. I increasingly found, on opening my eyes and rising from my seat, that my perception of the world and of myself had undergone subtle changes. Colours, textures, and shapes seemed to have become unusually clear and vivid; there was a refreshing newness, interest, and beauty in objects that had formerly been dull and humdrum; time seemed to have stood still, so that I lived in an eternal present moment -- while the effect lasted; and I felt as if I had been somehow purified of negative emotions and was radiating benevolence toward all beings.http://www.buddhanet.net/cmdsg/bucknell.htm
The person describing this experienced an increase of awareness of colours, textures, shapes and so on. But is this the awareness called sati-sampajanna? Is this what we should develop and stabilise? I do not think so. Still, in this description sati-sampajanna is suggested even without explicitly describing it. The author of the article (Rod) describes that he discerned a difference between how he usually is aware of colours etc and how he was aware of them at that moment he came out of sitting. Not the "living in an eternal present moment" but simply the recognition that now something was different than usual, a recognition that comes from being aware of how things arise now, at that moment he came out of meditation. And this, this awareness, is called sampajanna: it observes change in the present moment. Whatever arises it knows to be there and whatever does not arise it knows to be not there. Regardless of what ariseses and what not it knows what is there in every moment. Sounds easy but it is not.
Most kinds of awareness alter the other mind processes. When one sits and realises the mind has wandered from the object the mind stops it's wandering. When one become lucid during dream the dream changes, it stops for a moment, and one can alter the dream or wake up consciously. One is also aware during normal dreams but one only realises it was dream after waking. Like during emotional mind states one is not aware what happens at the moment but only in retrospect. Whatever had been happening the moment awareness knows it is already in the past and the mind stops the process.
This is different for sati-sampajanna: this kind of awareness does not alter the mind stream. It just notes whatever arises whenever it arises without interfering. It notices the mind wanders from the object during a sitting the moment it happens but the mind keeps wandering (and sati-sampajanna keeps knowing it). When the mind enters a state like the one described by Rod above sati-sampajanna knows it but it keeps knowing when that state changes back to being aware of colours as dull. When a dream scenery changes it knows it changes but it does not influence it (one can change the dream and other states with sati-sampajanna being present but that is, IMO, something else, a controling function, that arises from the presence of sati-sampajanna).
Sampajanna arises naturally for moments, especially when something new or surprising happens. The trouble is it does not last. The mind forgets to stay aware in this specific sense and falls back to it's normal awareness when it grows used to whatever arises. So it is very difficult to analyse and describe sati-sampajanna. To analyse and describe how it feels it needs to be stable for at least some duration. When sati-sampajanna is stable one can start to reflect back into it and analyse it and this is the point where terminology and analogy enters the equation.
For example sati-sampajanna feels spacious. One has the impression that everything arises and falls back inside some kind of space. Thus some describe it as spacious or call it "space" (a typical way to express this is the sky-clouds analogy). Trying to find borders or limits or a bottom from where things arise and fall back to one does not succeed - thus it is described as groundless or bottomless or infinite. But everything arises, exists and falls within it so some call it "ground of being" or "ground of existence" or some similar expression. Due to it's stability and not-changing in contrast to everything else some describe it as a "background". Because everything arises and falls inside it this awarenss itself is seen as empty (like space) and also empty of anything one can cling to, empty of anything that might lead to a lasting self-identity. Then there is the aspect of knowing that feels like seeing. One "sees" whatever arises the moment it arises. This analogy leads to the term "luminous" or "lucid" (from lux=light), because it is like a usually dark room (one's own mind) been lit up so one can see what is in it right now. What else? ... Ah, yes, when something appears it feels like a movement, something arises and a kind of spotlight shifts to it to illuminate it. Like a guard keeps waiting for someone to enter a door and alerting the king or castle when it happens: this is called "guarding the (sense) doors". Analysing this "spotlight" one notices that something moves with the movement of the objects, it stays with it, when something arises or the mind enters a new state this "it" moves with it - it feels like riding a horse or keep standing on a boat: A specific kind of balance is maintained even when the horse jumps or the boat is in a storm...
Probably I have forgotten some descriptions or do not know them yet - but all these analogies refer to the one and same kind of awareness: sati-sampajanna. Different teachers will describe it in different ways either by own experience or due to traditional analogies but this does not change the fact that the meaning is always the same.