If Iaido's Limit Break had a bottleneck, did ninjutsu have one too? What kind of bottleneck would it be? Why hadn't kunai throwing had one? Or had its bottleneck simply been too easy to overcome?
If swordsmanship, ninjutsu, taijutsu, and other techniques all had bottlenecks, he felt he should plan ahead and consider what those bottlenecks might be.
Swish, swish, swish-
Standing in the backyard, he suddenly drew his sword and fed chakra into it. The blade instantly turned pale blue and hummed as he slashed at the rock he used for Iaido practice. Three streaks of sword light flashed through the air, and the rock shattered into tiny pieces.
Then he sheathed his sword. He seemed to have gained some insight into Iaido's bottleneck. When he first started practicing, he had cut wood. Later, finding wood unsatisfying, he had moved on to rocks. Now he could split rocks with a casual swing.
He realized that to advance any further, he would have to cut something harder. This had little to do with the sharpness of the physical blade anymore; it depended on the sharpness of his chakra edge.
Even without attaching a chakra edge to his Chakra Blade, he could split rocks with a Chakra Scalpel, let alone wood. So the issue still lay in the sharpness of his chakra edge.
Once again, the inspiration came from One Piece. The swordsmen in that world had a stage called Iron-Cutting. If he wanted to push Iaido to its Limit Break, he probably needed to cut something harder. Iron-Cutting was the next step.
The samurai in the neighboring Land of Iron also had a technique that fired chakra as an airborne blade with a swing of their swords. It was an advanced chakra-control skill, probably a sword technique for Chakra Blades.