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Below are the rules of backstroke (Copied from The World of Masters Swimming). The underlinings are mine.
The first thing to note is that the detailed action of the stroke is not defined at any point so anything which is swum dorsal surface downwards and not underwater is legal backstroke.
So there is no such thing as "back crawl". While to many swimmers this may not be relevant but anyone who finds the back crawl stroke to be the hole in their IM should think carefully what this means.
Why do you still swim back crawl even though the backstrokers are vanishing into the distance, probably because everyone says that it's the fastest way to swim on your back; but is it? If you are a breaststroker or butterflyer (or better still both) this may not be so, have you actually tried it?
After one particularly depressing second length of a 200IM, I realized that I had to try something radical or abandon IMs. The first attempt at 50m on double arm or old-english backstroke despite scramble turns was still very close to my existing PB for 50 back. Since then I have persisted with the stroke despite coaches who cannot cope with the idea that anyone under 40 should do pre-1918 backstroke. However the argument is quite simple:
While double arm backstroke may not be the optimal solution to swimming the fastest possible backstroke times for the best backstroker. However for a swimmer (especially a breaststroker) whose primary targets are elsewhere and who has no aptitude for back crawl, then taking the time needed to develop an effective back crawl stroke may not be an economical use of training time and double-arm may be the better option.
Remember if you don't try you'll never know.
The basic double-arm backstroke action is of a double-arm pull with a reovery above the water combined with a breaststroke kick. The timing is actually rather similar to that of breaststroke in that the kick drives you forward as the arms are entering the water. Optionally a butterfly kick may be used as a second kick in the arm cycle (I generally use this in the 100IM or 50 backstroke events only).
Let's start from just before the start of the pull. The arms should be extended above the head with the hands together with the little fingers downward. The initial pull is an outsweep, the elbows should be as deep as possible. As the hands come level with the shoulders, the elbows should be bent with the hands only deep enough to avoid cavitation. The rest of the pull the hands make a downsweep to finish by the sides. The recovery is then straight-armed with the hands coming together just befoe the vertical and remaining together until they have entered the water. The primary kick is released towards the end of the recovery stage. The kick recovery comes right at the end of the pull or at the beginning of the recovery. The optional fly kick is done in the first half of the pull.
Remember this is pure heresy, but it might just work. If you're dead in the water on backstroke but have decent breaststroke kick, give it a try, you've nothing to lose.