In Eric Yakes’s book, The 7th Property, he explains the history and meaning of the phrase, “backed by”, and why it doesn’t apply to Bitcoin, for the same reason that it doesn’t apply to gold.

Once upon a time, bank notes were IOUs, entitling the holder to redeem the note for the specified amount of gold that was “backing” the note. That’s why the paper notes were said to be “backed by” gold. They were placeholders for real money. Real money was “hard money”, that took actual work in the real world, impacting the human economy, to create. Or, as you have put it, it required human action.

Then in 1971, Nixon ruggged the world by removing the redeemability of dollars for gold, turning them into pure fiat money, with no backing, only the political promise that the government is good for it. Whatever that means. Amazingly, the masses just carried on, pretending nothing had changed, even though the value of the “money” they were earning was now losing value on the whims of the political class of the day.

Like gold, Bitcoin is the thing of value. It’s what required hard work in the real world to produce. This work in the real world affected the human economy — people were paid for their labor, resources were acquired and put to use, services were consumed and paid for. That real work is what allows hard money to accurately convey price signal, to convey actual costs of things in the world — it is a feedback mechanism, part of the causal loop of economic reality. This is a necessary property of good money. Without that property, when money is just created from thin air at a keystroke, it fails to convey the true cost of things in the world; the price signal becomes distorted.

Bitcoin is the sort of thing that does the backing for promissory notes. It is not an IOU that is “backed by” something else more valuable. It is the money.