Thanks for all the thoughts. I was also thinking that this technique of hers was an advanced technique more suitable to someone with a high level of training. Her training consists of a two years of tae kwon do as a teen (20 years ago), and more recently she and her husband go to the shooting range a couple of times a year. She is not exactly what you might call SAS
I am not going to be able to talk her out of the knife... my sister is just that way. But I might be able to talk her into something more appropriately sized. Maybe not... honestly if she had a machete or a sword, I think she would use that !
I am not sure where she got this idea, but Habaz72 mentioned video games. Her son played a lot of games when he was a teenager, so maybe that was it.
Thanks again for your thoughts...