Translation
I am called Posta di Fenestra Sinistra (Window Guard on the Left) with my right arm tucked in close. We have no stability. One and the other search for deception. You will think I am going to attack with a downward cut, and I will turn my foot back and I will change my guard. Where I was in the left, I will enter in the right. And I am well placed to enter into the plays that come after.
Interpretation
Although they chamber their respective weapons on different sides, this posta is most clearly derived from posta di fenestra with the sword in two hands. The right hand directs the point of the weapon while the the left drives the power. As well as making cuts and breaks from this position, you are also well set up to deliver powerful thrusts. In this regard, it is functionally identical to posta serpentino lo soprano. All the players in the spear on foot section carry their spears from this posta, albeit on the other side of the body.
Interestingly, the spear on foot section also shows us posta de fenestra sinestra, although there the arms are crossed. Crossing the arms emphasises the defensive aspect of this guard when used with a polearm. By stepping through as you sweep aside an incoming attack, you naturally transition to posta breve la serpentina and easily flow onto any of the pollaxe plays.
It is this ease of transition and testing the opponent that Fiore refers to in the text when he tells us this guard searches for deception. His description of feinting from the left and entering from the right is a clear practical example of this in action.
Posta de Fenestra, regardless of which side you chamber it, is held in place by the arms and shoulders. It is this removal from the bodies core which puts it in the unstable category of posta. This guard is ruled by transition and mobility.