I find that when I'm doing Spinal Breathing Pranayama, I often have extra breath left to exhale when I'm already at the top of the spine. It's like it's too easy to go up past the curve of my back quickly, and then I'm soon at the brow. I stay there and the feeling almost feels like I'm going up past the brow outwards as I keep breathing in. It doesn't seem to have a definite end...
and I remember reading in either one of the "Secrets of Wilder" or SBP books by yogani that the spinal nerve actually extends out forward and upward past the brow. Is that factual or fiction? What should I think of that?
I've been trying to pace better when going up/down the spine so that I don't go past the top end like that. I'm just wondering, what happens if I keep doing that?
The sushumna nadi extends out beyond the brow. That's fact not fiction. So it is fine to allow the attention to be drawn out further than the brow during SBP, towards the end of inhalation, if that is happening naturally.
As for what will happen if you keep doing that, well, quite a few things and they are all good.