Talkback: How the 'Ocean Walker' swim stroke can benefit triathletes
Total Immersion springs to mind?
Re early hand entry; I don't doubt that there's an advantage to be gained from this but I think there's some twisted logic applied as to why it's done . Taking the analogy of diving off the block, the body moves quicker thru the air than thru the water. Once in the water, the body moves more efficiently under the water than on top due to disturbed hydrodynamics of swimming on the surface. So, rather than considering the resistance the arm generates in the water (which will be more than in air) we need to consider the lift it generates.
The benefit of early hand entry & extending the arm underwater is most likely from the lift this produces (like a planes wing) so the swimmer doesn't have to/need to push down to keep the body on the surface and therefore can devote more effort to going forward... Probably.