This is a brilliant cast, especially when fishing heavy bugs for Grayling, although it can be used successfully for several species. It is very useful for getting your bug down deep very quickly whilst at the same time eliminating drag which is introduced via your fly line touching down early in the cast.
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 make a high (not low) back cast during your overhead cast, your fly will come directly over the rod tip. Add to this a firm stop (from a reasonably high position) at the extremity of the forward cast, coupled with a slight pull back of the rod tip, "what will happen"?
The end section of your fly line, leader, then fly, will progressively increase in speed, causing your fly to come to an abrupt stop, and because it is quite heavy, it will kick over and tuck down towards the water much earlier than your fly line does, thus plopping in the water (which can be an attraction in itself) with some meaning and therefore sinking much quicker than it 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 away almost immediately.