Translation
Of the fourth remedy master of dagger I am a counter remedy. And the two plays which come before, I am also doing the counter. In this way I will break the hands of the master with a move I will make quickly. If he were well armoured, I would ruin them without doubt.
Interpretation
The counter to the 4th master uses a wrist lock. The play begins by making a fendente attack against your opponent, which they jam using a sword grip against your forearm as the 4th master. You will be unable to push your arm forward, and will struggle to pull it back.
Use your left hand to clamp the dagger blade against your forearm. Although you might catch the thumb of their left hand, you are really targeting the right wrist. Lay the blade across the wrist itself, rather than the back of the hand. This will cause the wrist to bend, breaking their structure.
Whereas your opponent is perfectly positioned to flow on to the play of whichever scholar they choose, you have quite a distance to cover with your left hand to reach the dagger. You will need to realise the situation and act on it much faster than your opponent. This is the moment shown.
Step forward with your left foot. Drop your right elbow down to your hip and roll the forearm over so that your right palm faces downward. You are pulling your opponents wrist into your core. Lean into them slightly, cranking the thumb down.
There is a great sense of feel to applying a wrist lock. It is a difficult sensation to describe and requires a degree of practice and feedback from your opponent to get the knack of it. The difference between an uncomfortable twist and an unbreakable lock is a subtle one. Key to making this work is having the fulcrum of your lock (in this case, your opponents wrist) pinned right up against your core. That not only gives the lock stability, but also means that your body movements transfer directly into leverage on the wrist.
Your opponent will drop to their knee at your foot. Their left hand may be free and making some kind of cover. With your left hand, sweep any shreds of defence away. You will have a clear strike straight down where their neck meets the left shoulder and into the top of the lungs.
You can see different applications of similar wrist locks in the following plays.