Re: "blocking" with japanese sword
The back (non-edge) is soft precisely because softer steel does not break, it bends, and can then be repaired. The edge is hard, and therefore brittle. The back and sides of a blade can be repaired, but once the edge (inside the hamon) cracks or chips, it can never be repaired (although the blade still works). Sword makers would make swords completely soft if they did not have to hold and edge. Some schools use the back of the blade for parrying, but I believe most schools use the "omote" side of the blade, which is the side that is facing away from you when carrying the sword.