Watching Crazy Samurai Musashi (thanks Dro!), I realize that I have overlooked a currency in combat: weapon speed.
Each margin of success of the Block action should reduce your opponent’s weapon speed by one.
Being staggered should reduce weapon speed by one.
Being knocked down should reduce weapon speed by two.
The penalty lasts until the character takes a non-Strike action.
Weapon speed X weapons are immune to this penalty.
It’s a small penalty that should create unpredictable moments in combat.