Translation
When I saw that there was nothing the sword could do to you, I immediately took this grappling hold. I believe, see and feel that your armour is not worth anything now that I have you in the strong lower bind. In the next picture, I will show you.
Interpretation
Having made the crossing of the master play, your sword and left hand are pointing towards the centreline of your opponent, as shown by the drawing of the 1st scholar. Due to your opponents armour, however, you can see that continuing the attack with your sword will be inneffective.
Step in deep with your left foot. Extend your left hand under your opponents elbow, and reach behind their shoulder. Keep your own sword hand back at your hip out of the way to give the technique some space, as shown in the drawing.
Pivot on your left foot, arcing your right foot around behind you in a clockwise circle. Your left foot should end up level to or slightly behind your opponents right foot. Your hip should be pushing against theirs. Keep your left elbow locked against your hip and lever your opponents shoulder down into a ligadura sottano (lower bind).
There are numerous examples of the lower bind throughout the Fior di Battaglia. You will see it used in the following plays.