This is a brilliant cast, especially when fishing heavy bugs for Grayling, although it can be used successfully for several other species too. "Why such a good cast"? because it is very useful for getting your bug/s down deep very quickly whilst at the same time eliminating drag which is often caused by your fly line touching down too early in the cast.
"So how does it work"? Well, bearing in mind that the fly line will only follow the path that the rod tip takes during your cast, and therefore the leader and fly can only follow the path of the fly line, if you now send the rod tip higher than usual on the forward delivery of a conventional Overhead cast and incorporate a very firm stop, coupled with a slight pull back of the rod tip, your fly, or flies will still come directly over the rod tip as normal, but this time coming to a very abrupt stop at the extremity of the forward cast.
"So what will this do"?
"Yes you've guessed it", as your fly line progressively increases in speed this high abrupt stop will also cause your leader, and more importantly your heavy flies, (which is what we would generally use when employing this technique) to also turnover abruptly, causing them to tuck down sharply towards the water much quicker than your fly line can, thus plopping into the water with meaning (which can be an attraction in itself) and therefore sinking much quicker than they normally would, reaching the fish very quickly. This can be a great bonus if you only have a small pocket of water to fish before the faster current takes over, which in normal circumstances would drag your bug/s away almost immediately.