People are very much affected by what they imagine is going on. For the unbendable arm you don't tell people to extend their arm effiiciently, you have them imagine the arm extending out to infinity, or imagine the arm as a firehose.
I'm not sure why any of this works-- it may have something to do with activating one's own mirror neurons, but I do think the placebo effect should be viewed as a special case rather than a thing in itself.
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