BTW it was shared on another prepping site so it seems the conversation and then this bit of info happened to all fall together. Oh and the small stone is simply worked in a circle around the rim of an upright can thus no spillage."
I personally found it interesting that shortly after seeing this video for the first time, I learned why this works? It is the way they flange the lid onto the can at the factory. The top actually wraps around the body of the can, like the picture below. When you use the concrete or rock, you are simply rubbing off that top layer. Cool!
I still store a lot of can openers, since two is one and one is none.