Translation
I do the counter to the thirteenth play. His hands have gone away from my face. And because of the way I have done it and hold him, if he does not go to ground, I will take great disdain.
Interpretation
This is a curious play in that although the text gives no real details and instead relies on the pictures, the Getty, the Pisani Dossi and the Florius all show the play being performed slightly differently. The Getty shows the grab on the players right arm although the 11th scholar has said to grab the left. The Pisani Dossi and the Florius show the grab to the left arm. While the Getty and the Pisani Dossi scoop out the left leg, the Florius scoops for the right. It is difficult to know exactly what is intended, and the interpretation provided is one of several possibilities.
Pisani Dossi 
Florius
In the play of the 11th scholar, you had grabbed you opponent with both hands around their waist. Your opponent slid their hands up the centreline, and using one to suport the other, are now trying to drive them into your face.
Drop your weight, move in close, and scoop out the players right leg with your left hand. Try to catch it as they are moving forward. This is the crux of the play and needs to be done with a high degree of assertiveness. Bend your left arm and twist your hips to really get this working. You want them to be left standing on one leg.
Dropping your weight like this not only lets you reach the leg, but also moves your head out of the way of their attack. As you drop, raise your right hand to catch your opponents left elbow. You should find yourself in the position shown by the Pisani Dossi manuscript.
Scooping the leg like this will turn your opponent in an anticlockwise direction and cause them to start falling to your left. Exaggerate this by locking your left arm, and by extension your opponents right leg, against your body. Pivot anticlockwise on the balls of your feet. As you pivot, use your right hand to push your opponents left elbow across and then down.
You will end up having turned nearly 180 degrees and your opponent will be falling on their back with the leg still captured. Although the application is different, you will also see the same principles of this throw being used in the 6th scholar of the 5th master of dagger