I was working on a simple upward climbing piston contraption, and in doing so, stumbled upon the strange solution in the thread attachments. I didn't follow any kind of YouTube tutorial, so if someone else has already created the same contraption, I wouldn't know.
It activates after a block is placed in front of the bottom observer, if you'd like to try it for yourself.
I'm no redstone expert but I played around with it and think I've got it figured out.
To start with, the bottom observer is only needed to generate a pulse to start the contraption, it works as well with the observer placed on some other side of the piston so it gets left behind, or it can be connected with redstone dust and your version can even be started by knocking out the observer.
So the bottom piston gets its pulse and pushes everything else up a block and immediately contracts (so that it can be moved).
Now, moving the upper observer generates a pulse (I've verified that by pushing an observer up to a piston which briefly extended).
That pulse makes the upper piston push down the slime blocks, redstone block and observer(s) and pull everything including the bottom piston up a block.
Then, and I can't claim to understand exactly how that works, it appears that moving the bottom piston into a space that is next to where the redstone block just was causes it to be briefly powered, which restarts the cycle.
I think I may have been off base regarding how the bottom piston is powered.
It seems that a piston can be powered by a redstone block diagonally above it, but it needs an update.
So my new supposition is that pulling the piston up provides the update, it discovers the redstone block and extends, which pushes the redstone block out of range so the piston contracts again.
You can even build it entirely without the bottom observer and start it by just placing a bit of redstone dust, or just about anything, next to the bottom piston to provide the update to start it.