Translation
I made the cover of my master and immediately with my left hand took his in this way. And with my dagger, I stab him in his chest. And if my dagger was not sufficient, I would do the play that follows.
Interpretation
Against a sottano stab, grab your attackers wrist and forearm in a sword grip as shown by the 9th master. Keep your elbows close to your body, but still sufficiently extended that the blade has room to clear. Turn your hips slightly clockwise directing the attack off past your right hip.
At the end of that turn, let go with your right hand. Use the motion of your hips to carry your hand back. Smoothly draw your dagger, leaving it chambered by your right ribs.
If you were already holding your dagger at the start of the play, the deflection can also be made with a single hand. In that case, keep your dagger chambered through the deflection ready for the counterstrike.
Lock your left forearm across your hips, pinning your opponents dagger out of the way. Sink your weight onto your front foot and drive forward with your right hip, punching the dagger into your opponents chest. This is the moment shown in the picture.
If, for any reason, you cannot draw your dagger, transition this play into the next one, and finish as the 9th scholar.