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You mean you don't see people that have a mental breakdown rocking back and forth? It doesn't help it's a thing that happens!

Stimming helps autistic people. Refraining from stimming leads to increased anxiety in autism [1].

It can very well be the case that it does not help any allistic people, I'm not familiar with the literature in that regard. Searching for "stimming PTSD" in google scholar does not bring up any studies that look into this, so if PTSD and ASD share some common characteristics this might be worthwhile looking into.

[1] https://journals.sagepub.com/doi/abs/10.1177/136236132311951...

It does, some PTSD therapies explicitly teach people with PTSD how to stim.

Well if ptsd is being stuck in a repeating loop in itself...