The term soft is in relation to the crank which is WAY harder than the rods or piston which are both made of aluminum! So what i am trying to understand is regardless of the speed of the piston and what not, when you are at the point when the flex is supposed to happen for the crank to flex means that the rod and or piston is fully flexed/stretched?? reason being is that it seems to me that in order for the crank to flex via the force of the combustion cycle all of that force must travel through the piston and rod before it even gets to the crank, therefore THEY take a fair amount of the abuse. Now with the crank pin sticking out with no support obviously makes that the weak link more so than the say middle of the crank, so i can see how an overhardened crank can lead to destruction. I have been machining metal for over 25years from aluminum to inconel, nitronix 50 and anything in between! So what you know in nitro engines i know in the machine process of creating them( my 4 axis CNC mill is running as i type this!!)! taking a wild guess but i assume the pistons are a form of 6k series aluminum (6061) rod a 7k series (7075) the bushing are more than likely made of a self lubricating bronze-oil lite and since the cranks are being hardened I am sure they are some sort of carbon based alloy but I am not sure on that. Now taking all of that into consideration the crank is easily 10-15 times harder (maybe more) than the aluminum parts and being the "caboose" on the line of fire is why i am having a hard time seeing the crank flexing (not moving). I think the reason the over hardened crank broke is simply shear force and shock and the softer crank can absorb that shear force/resonance. For something to flex and break implies that it bent before breaking and everytime I have seen a crank break it is sheared off straight at the pin. Make sense? I am not saying that the crank never flex's but the amount it does is multiple times less than the rest of the assembly.