This diagram shows us a possible way to make the Issen function, although I'm not sure if this is the exact method used by Capcom in Onimusha.
By putting two bones on the weapon of enemies, we can use the first one to trigger the Issen state when it hits the player, and the one following deals damage to the player.
So if players hit the attack button between the time of Issen Bone and Damage Bone, they will succeed in performing an Issen Attack.
And by tuning the distance between these two bones, we can tune the difficulty for Issen on each individual enemy.
