Hi,
Re "i dont understand when, how or why this kind of behaviour seemingly transcends from normal to autistic?"
My understanding is, any of the Autistic traits can be present in anyone - neurotypical (NT) or otherwise. What identifies them as "Autistic characteristics" is when they occur clustered with other Autistic characteristics, and can be stronger in us than in NT people.
So, for example, my friend from school has very strong interests in one or two subjects, and she really immerses herself in tudor history, and modern languages. When she's discussing these subjects it's hard to get her to stop talking! But she is very socially confident & competent, very naturally empathetic, and in many other ways clearly not Autistic.
On its own, no behaviour is uniquely Autistic.
Regarding movement based characteristics, all humans have a need for sensory stimulation, and everyone has their own degree of hyper / regular / hypo sensitivity to different stimulus, and so may carry out different stimulating or calming actions. Again, on their own they don't indicate Autism, but if they are part of an overall picture that includes difficulties with social understanding or functioning, and narrow & intense focus on specific interests etc etc, then they may be indicative of Autism.
Does that help at all?