Well that is true but the "unbreakable" aspect comes from the differential temper as well....coating the spine in clay and on the sabre grind sides gives a cooler temperature on those areas when heat treating the blade...so the hard carbon added to the middle of the folding process means the edge can take a high Rc heat treatment but the sides and spine become differentially tempered by the painting of clay on the blade.

Each swordsmith would have a different painting stroke and this adds to the steel giving the "hamon" to the blade...it acts like a "signature" to identify the swordsmith...and by having a lower heat treat on the spine it could absorb strikes from other blades when blocking and moving the opponents sword...but a full on strike to the spine has caused the sword which is struck to break...as have full power clashes of edge to edge...that is why a Samaurai would learn to cut with the blade on the back stroke....bringing the blade back towards him...the parry and thrust is done with the edge pointing down...Aido is a cool sport if you like Samaurai swords and want to learn how they were used...I think there are a number of Sensei masters on Hawaii for Aido...but the spelling can be varied in different country's...it is phonetic from the Japanese....and means basically Sword fighting...