How does the stem-loop cause intrinsic transcription termination?
There are two mechanisms of transcriptional termination in prokaryotes.  The one shown here is "rho-dependent" because it involves rho, a DNA-RNA helicase that loads on and unwinds the RNA from the DNA, terminating the elongation by the polymerase.  Check out [1] which shows a model for how rho multimers move through the RNA.

The other mechanism involves just a hairpin. The paper [2], especially Page 23, part C, shows how "rho-independent" or "intrinsic" termination works.  Basically, the hairpin slows down the polymerase and destabilizes the transcription bubble, allowing the intrinsically-weaker A:U bonds to fall apart.

There is no requirement for a hairpin in rho-dependent termination, and I think the movie is mixing them up.


 Geiselmann J, Wang Y, Seifried SE, von Hippel PH. 1993. A physical model for the translocation and helicase activities of Escherichia coli transcription termination protein Rho. Proceedings of the National Academy of Sciences of the United States of America 90: 7754–8. [pdf]
 Datta K, von Hippel PH. 2008. Direct spectroscopic study of reconstituted transcription complexes reveals that intrinsic termination is driven primarily by thermodynamic destabilization of the nucleic acid framework. The Journal of biological chemistry 283: 3537–49. [pdf]