If my Latin memory serves me well, ostension is from the verb ostendo (ostendere, ostendi, ostensus), to show/display/explain, and acting something out is simply displaying something. Thus, ostension becomes unwittingly acting out or mimicking the greater part (if not the entirety) of an urban legend that is already part of the body of lore. And yes, both extension and ostension (as well as retension, intend, subtend, distend, tense, etc.) come from the Latin tendere (to stretch).