Translation
I am positioned in Posta di Fenestra Sinistra (Window Guard on the Left). If I do not wound you with a thrust you will be lucky. With the point held high and the arms low, I will bring the back foot to pass off the line to the left. I will put the point in your face for you are without any defence. The play that comes next is how all three masters finish. If you try it once, you do not want to try it anymore.
Interpretation
Posta di Fenestra Sinistra is simply a mirror image of Posta di Fenestra Destra, and is used in the same manner.
This is a rear weighted stance with the right foot extending forward, ready to slide to a new position. Your left hand is on, or slightly behind the balance point of the spear. Fiore shows the left hand being held behind the head, although moving it forward slightly so that the knuckles of your left hand rest against your cheekbone will give you slightly better speed and control. The right hand crosses over the left, and controls the spear by moving it around the pivot point of the left hand.
You look down the length of your spear to your opponents face. Your right flank appears exposed, inviting an attack.
The defence of this posta, as with all the spear masters, is exchanging the thrust, as first explained by the 8th scholar of the 2nd master of sword in two hands.
Against a thrust, slide your right foot across to shoulder width, and either forward or back to give you an appropriate distance. Strike with your left hand straight down the centreline. Pull your right elbow back to chamber your hand at your hip. This is something of a scoop, which will catch the incoming attack with the lower half of the spear shaft and direct it offline.
As with all deflections, there is no need to push this further out. Once your opponents point has passed you, you are safe. Be sure to keep your right elbow tucked in. As well as creating better mechanics, it also removes the possibility of it being hit with the deflected spear point.
Step through with your left foot. Drive your hip anticlockwise and punch out with your right hand, using it to power a counter thrust to your face. Keep your right hand low so as to maintain your cover. From posta to counterthrust is all done as a single smooth movement.