The soft back - and even the edge quench - are both "surrounded", in part, by the performance tests that are designed to demonstrate that a blade smith can determine and control which areas of the knife he/she want hard, soft, springy, etc.

Don't forget that those performance tests require that blade to shave, slice, chop and bend while retaining an undamaged cutting edge and without breaking at 90 degrees. That would be difficult to achieve with a fully hardened blade. Not impossible - but difficult.

Just think of all those ancient battle blades and swords of generations of Asian soldiers that were clayed to soften the back and leave the edge hard. (Use some imagination there.)

The process is neither new nor unused.

Following my successful performance test(s) I seek to achieve fully hardened blades if I am not seeking a hamon or I am doing stainless San-Mai.

I think the whole process of differential tempering or hardness is as old as steel blades. The ancient smiths of the migration era in Europe through the Vendel period and into the Viking age all made blades in this manner. Whether it was from "piling", or forge welding a hardenable edge steel onto a soft iron back or core bar, or it was a process of edge quenching, we have been experimenting and using, this technique for centuries. When it comes down to the ABS test blades, you don't have the opportunity to pile steel onto unhardenable iron. For the JS blade, you have to use a single bar of homogenous steel. For the MS blade, you have to use a 300 layer pattern welded bar. The test becomes a way to determine whether the smith  has "got the heat treating chops" so to speak. 

As for the history of the ABS using this in the test, I am not sure when the test blade was adopted. Maybe Kevin will chime in with the history or maybe someone will ask Harvey Dean....... 😀 

Both Karl and Joshua put a great deal of information into their replies...and I found myself reading, and nodding my head in both.

   WAY back, when I was a "baby" Bladesmith, I asked Bill Moran about this very subject, and although it's not verbatim, his response was something like...

  Its a way to show that the Bladesmith knows and understands steel, and how to heat treat it. He also said that without a differential heat treat, most would never be able to pass the Journeyman smith testing, let alone the MS testing. 

  One of my mentors, and actually the first person to be awarded his MS stamp/rating under the system we now know/use, was Wayne Goddard.  Wayne always emphasized to me, the importance of knowing and understanding heat treating.  I always got the impression that he was never a fan of the differential heat treat, but he did use it, and knew/understood it's purposes.

  As far as how/who "invented" or "rediscovered" it, I don't think either.  Wayne always said that Hard edge/soft back was a matter of course in history.  Hardenable steel was rare and what little was there, was often used only on the "business end" of tools and/or weapons.   This was/is supported by my own research on the various "blade" cultures.  What we call "soft back draw" these days was simply a by-product, created via the scarcity of hardenable steels, and eventually was recognized by warriors and smtihs as an attribute to the durability of edged weapons as it applied to the various times/points of history.  Again, my research infers that once it was recognized, it became a prized attribute in just about anything with an edge, that required durability.

  Maybe the old saying.. "Everything old is new again".. is true? 😉