Very interesting comparison between a sword and an arm. An A-HA! moment for me.
One of the theories/principles I am currently proving to myself in training.
The sword's sharp edge can be broken down into 3 1/3rd parts.
The 1st 3rd from the tip is for killing
The 2nd 3rd in the middle is for control
The 3rd 3rd from middle to tsuba is for defense
I agree with the relationship between sword and arm which helps make taijutsu techniques more effective - e.g. try nikkyo with uke's wrist out in front of you away from your body, without and then with a shinai or bokken.
Where I don't agree, is the purpose you assign to the different thirds. The first third from tip (also called monouchi) - while it can be used for attack, it can also be used to defend, or indeed defend at the same time as attacking (defence by attack). For example, some people perform the following technique as a defence move, whereas I understand it as attacking the attack, and thus defending:
(and to be clear I see that as a good example - clear intent to cut the attacking sword)
The challenge as I see it, is to be connected to the sword at all times (connect tanden with sword, first imaginatively and, with practice, a physical sensation). Often you focus more on one point than others - the point of contact - or the "meat of the bat" - think cricket if you're a Brit, baseball if a Yank
- which is roughly one third in from tip.
A lot of people during suburi, tend to lose that connection - the sword goes dead at one or more places - often during the swing when it's behind them.