We saw Star Wars: The Last Jedi for a second time a while back, mainly because a large group of friends was going and we had most of the theater to ourselves, so it was sort of like a private screening. During my re-watching, I spotted a number of things that I'd missed the first time, but one in particular stood out…
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And while those things do matter, I think they're secondary to the main purpose of the "blood salt:" To let you know that Luke is not really there for his battle. Watch carefully, and you'll see that Luke doesn't leave footprints as he walks out to battle, nor as he fights Kylo Ren in the requisite lightsaber duel. Instead, he's projecting his presence via the Force.
I'm sure this was revealed in numerous spots on the internet, but I hadn't read any spoilers (even after seeing the movie), so it was a neat surprise on my second viewing.
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