Blake M:
Sure. I would describe it as a feeling of the death of the "I-thought" that is talked about in advaita. In more practical terms you know that constant feeling that we have of having a separate self? For a few moments I felt absolutely no trace of a seperate identitiy. I then immeditately asked myself, well if I'm not an "I" then what is all of this, what is the nature of reality? and so I saw the Big self, which I would say felt like seeing God.
All of this seems very abstract to me. I was hoping you could talk about sensations and perceptions. I don't know what it would feel like to see god or the big self. Those words don't have much meaning to me. I'm also not exactly sure what you mean by a separate self vs no separate identity. For example, did you look at a teacup next to you and say "Ah, I am that teacup!" Or was it a sensation that you were embedded within your experience rather than observing it - a veil over reality had been lifted. If you looked at your hands in front of you, was it still logical to say, "these are my hands," or were there no hands at all and just a field of meaningless colors?
Blake M:
I was paying attention to the images on my mirror infront of me and it felt like I was the building blocks with which those images were being manifested. and the nature of those building blocks was like a great happiness or contentment born from great understanding. This i guess could be characterised as experiencing "buddha nature" in everything. Seeing this great happiness and understanding is what caused me to bust out in laughter. After a couple moments I got excited by this experience and began to see VERY subtle senses of self creeping back in, trying to make an identity from the experience. In fact it was so subtle perceptually, that I noticed it more from the accompanying suffering that came with it than the sense of seperate identity itself.
So, when you looked in the mirror, what was the perception itself? You saw yourself as building blocks - what do you mean by this?
This sense of self that crept back in, what was it? Was it a feeling of control or "agency" that suddenly became apparent as it crept back in? Maybe by looking at how it came back you can see what it was that left in the first place?
Blake M:
To be honest Im not sure why it faded, in the little that I learned of zen, these flashes of no-self/enlightenment can occur many times before a true enlightenment so it is natural for them to fade. I would emphasize that this wasnt a mindset, even though it probably sounds like im nitpicking, the mind didnt have anything to do with this. My mind was still, it was a change in the experience of my perception from duality to a nonduality.
How can you be sure this was a non-dual experience? I've had a lot of trouble parsing the terminology on these things. I've had a number of momentary perceptual changes that I could label as "non-dual", maybe you can help me figure out what's what if you have more zen experience. Did you have a teacher that went through this with you to confirm it was a satori? If not, could you point me to some teachings or resource that most closely matches your experience or defines the terms in a way you'd agree with?
Blake M:
I havent tried yet but Im confident i could get back to it. It feels like the small self is less heavy or incharge right now so Im assuming that would mean there is less illusion to get out of the way before seeing reality truely again.
Good luck!

If it happens again, maybe try watching the transition in and out of it closely to see if it offers any hints as to why it manifests.