This is how I felt it coming on..
My first 'true' narcotic experience was at 55m at the Blue Hole in Egypt. I had previously been to a similar depth in Malta so I was confident that I wouldn't have issues. Also I was vetting the instructor who was going to be my Trimix instructor so he was guiding.
I don't really remember what happened. One moment I'm fine, then the next we've gone into the 'arch'. It's a pretty large tunnel, but the slight change from natural light to gloom made me loose it.
This is how it felt during...
It was as though I was watching myself drive from the backseat.
This is how I knew i was narced...
I don't know if I knew, but when the instructor pulled me down from the tunnel ceiling, that was a pretty good indicator.
This is how I felt when it was ending, or afterward...
The dive felt as though it was a dream afterward. Even though I got back into open water and completed my stops easily, the whole 10 minutes in the arch was .. vague.
I've since done the 'arch' on Trimix and the dive is totally different. When I'm narced everything seems to have a purple tinge to it, which is not wholly attributed to the light loss, as it doesn't seem to be there on TMX.
I know that I never want to feel that again. The loss of control was frightening.