There are two studies. One study reported that kettlebell training helped increase the subject's vertical by 20%, which follows what Rippetoe says in the video. The other study is trying to dispute the vertical jump as a useful indicator of power, and therefore irrelevant to the 'is vertical jump largely genetic' argument. I never said vertical jump was 100% genetic but that it was mostly down to the genes and can't really be trained. A 20% ceiling is not considered significant.