I will answer this first, so it's clear that I'm working on these answers myself, and not asking someone else to fill in my blanks...
Here are my current thoughts/answers, subject to change as I learn and grow, of course!
When you practice Sanchin, does your strike originate from your hips, your core/torso, your feet, etc?
I had thought the strike originated from my hips, with a snap forward powering the strike; however, I think it is more of an all body interaction that truly makes this strike effective and smooth.
I'm currently working on contracting my core to "throw" this strike out, but I'm not sure I've got this right...more practice and work to be done!
Is this origination the true source of this strike's force?
I think the origination is from my feet, but I'm still working to feel it correctly.
Do you practice the movement with tension througout, or with looseness until the last moment before impact?
I've tried both. I feel like it's stronger when I can relax until the end, but this is harder than it sounds. I think I need to work on keeping my shoulders down..again...work in process (WIP)
Do you practice as kata and form, or do you also practice against an opponent or punching bag (or some other striking surface?...Have you ever tried sanchin in water?
When I had access to a pool, I did this in the pool, and it helped me "feel" my movements. Now, alas, the pool is a memory, and I am simply doing on land. I have been doing simply as form, but I plan to do both as kata and against a bag tomorrow when I go to the gym. I think I need some kind of impact feedback to understand more of what I'm doing here.
Does your shoulder have a significant impact on this movement, or is it along for the ride?
Right now...I think I involve my shoulders too much, as I don't think my shoulder should be so involved...at least, no more than the rest of my body. I don't think I should be throwing this from my shoulder, but they keep trying to butt in to the whole movment