Question about the first guard in Fabris

Had our first question about how to approach some of the interpretations of the plates:

My partner asked:

ok, first question for you.

p. 35 (first paragraph) "in this manner your opponent will not be able to come in above your sword; and that part being the weakest, it should be the most strongly defended".....my question... why is this part the weakest.

I thought about it, and read over some of the manual:

Just looking at the formation of the sentence, "that part" seems to refer to the target area you'd be defending. If someone is attacking above your sword while you are in this guard, they're attacking the head, and possibly the arm (he mentions earlier the possibility of the arm being the target). Looking at the plates, where exactly is the forte really protecting? He mentions the guard helping protect the body more efficiently, too.

Check out page 25, where he talks about why to hold the sword extended: "If your opponent simply places his forte to your debole and goes for the attack, he will find it difficult to succeed by virtue of the small opening that is one of the properties of the manner of holding the sword." Also, check page 28, talking about posture: "If a person could make himself so small as to be able to cover his entire body with the forte of the sword, it would be ideal. But since this is not normally possible, you should at least make an effort to cover as much of it as you can, so you can enjoy more safety."

Does that make sense? I don't think he comes right out and says what the weak part is, but he's already said this is an effective guard against cuts, and it appears the primary cut someone would attempt would be from up to down, near the head or shoulder.