Not to discredit the entire technique but even the woman who originally posted all of the inversion instructions and testimonial videos on YouTube only had average length retention over the long term. I haven't heard of anyone getting 12+ inches of growth in a year unless they would have gotten about the same growth no matter what.
I don't think that there is any harm in trying it out especially if you would be happy with modest results. However, I wouldn't rest my hopes and dreams on it.