Strain. When you swing a sword you have to flick your wrists in the process. Like in casting a fishing rod. [And he'll take a step back out of Elicia range to demonstrate. Then he moves his hands together and does it again, exaggerating the awkwardness.] Doesn't work right when you hold it wrong, does it? Kinda choppy. And since you have to work harder to make a cut and the cut's not smooth, [Putting down the sword, he never unsheathed it and touches the inside of his wrist.] everything in here gets all jerked around and stressed after a while.

Same when you block another sword. That sword will push your sword back, twisting it out of your grip. The more you hold it wrong, the more difficult it will be to hold on to your sword. And that will make your hands, and especially your wrists, smart.