A perfectly normal katana can easily cut bamboo, but if you set the blade against the bamboo instead of taking space to build up the swing, it's going to take a LOT more strength and leverage to cut through it. Speed is awesome where there's space to build it up, but Mifune's sword doesn't have that space. The chain is wrapped airtight around the blade, and it isn't going to slide up and down. I'm also going to quote you for my next point (though the emphasis is mine).

[quote=drallinix]
The remaining part soared off into the distance as Mifune [i]drove[/i] his sword through the remnants of the chain pinning it to the ground.
[/quote]

The word choice here is particularly evocative. When I say my character drove his sword through someone's heart, do you picture a light prick of the skin that just grazes the heart itself? Of course not. You picture a sword being shoved violently clear through someone's body, and likely out the other side. 

Since you never specified how hard it was or how deep the sword went, it's the common definition that takes precedence. You know this already. We've gone over it half a dozen times in our fights. Mifune drove the sword through the chain, which means that it's not resting lightly on top of the ground. It's at least a couple inches into the dirt, and the chain has a solid grip on the blade. Speed is Mifune's thing, but strength goes to Herriman. He has more power and more leverage to work with in this situation. 

I have every confidence you can win, but not if you keep making the mistake he did and trying to combat his strengths with your weaknesses. He learned from it when we told him where he was going wrong and now he has the upper hand, but I've seen you do that before too. This fight has the potential to be much more amazing than it already is, but you can't stop taking him seriously if you're going to win.

To sum up, you made a mistake and handed him a pre-set trap, all he did was take advantage of it. It'll take a lot more than a flick of the wrist to get out of it.