Try Brian's Vimeo videos (just go to Vimeo.com and search "Brian Manzella"). There are two fairly nondescript videos on there titled "gm" and "Mike Golf Walk." In those videos, Brian addresses the "tumble" to a certain extent, and the motion and description are slightly different than in the "Below Plane" youtube videos. I think they give a slightly different twist on it, and it helped it "click" for me a little.
Whether I'm doing it correctly or not is another matter entirely.
Whether I'm doing it correctly or not is another matter entirely.