This tactic is completely unnecessary if a trainer introduces a girth and then a saddle slowly, does not scare a horse with it and does not let the horse learn to buck as his reaction to a girth and a rider.
But, when a horse has already learned how to buck and has gotten quite good a bucking people off, this method is very effective in teaching him 'after the fact' that bucking is not a pleasant trip for him.
Ray Hunt was the first person I saw using this method and as far as I know, he only used it on spoiled horses that already had a bucking problem. I believe that is the same with the modern clinicians that I have watched. Once a horse has learned to unload riders, you have to do something to 'break the cycle' or you have an unsafe bronc that bucks a rider off anytime it suits them better than being ridden.
When I used to get in so many spoiled horses to retrain for the public, I did this to the confirmed buckers. Without some good reason to quit bucking, they were headed for the slaughter plant anyway, so I guess it beat the alternative. Two that it did not work on headed to the PRCA (RCA back then) rodeo string and one of those was still bucking 10 years later. Most buckers just buck hard enough to get their average riders of but not hard enough to get paid for it.
Like all other 'pressure and release' techniques, it requires perfect timing. You pull on the flank rope as long as the horse is bucking and kicking and the instant he quits and just runs, you quit pulling. You have to do this several times on confirmed buckers. You do it every time you saddle them up for a while. You can give it up when they give it up. When you saddle them up fresh and they just let you longe them by the flank rope without bucking or kicking, they are usually done with it.
BTW, 'yellow editing' is when someone with an agenda edits out bits and pieces of several tapes to make someone look bad. This is something that HSUS and PETA are very good at doing. You can make anyone and anything look very bad if you take bits and pieces and put them together with an agenda.