Dear Sirs, with your indulgence,
I am not a regular forumite, but I think I can offer some insight into the mystery of Bicornio.
Brendan Heslop has posed an interesting question, but the answer appears to be quite straightforward if you look at it in context with the basic foundations of Fiore and his longsword system.
Posta di Bicornio allows very quick transitions from the centre-line to an attack from either Posta di Fenestra Destraza or Posta di Fenestra la Sinistra.
As Fiore states, there are two horns from this attack, which can come from either of the Fenestra, and they are most effectively thrusts, hence the horn metaphor.
The reversal of the left hand is a most efficient starting point for the quick rotation of the sword when transitioning from the posta to the attack. It also allows the arms to be chambered close to the chest for a strong forward motion.
Furthermore it allows for fluid movement between those postae which sit on the low line, for example.
It is perhaps worth a little speculation that Fiore, akin to his use of the Bestiary to symbolise the essential natures of the art, may likely be alluding to the actions of the bull for Bicornio. Other than a ram or rhino, I cannot picture another common two horned member of the animal kingdom other than that found in the bovine family <img src="/forum/images/icons/smile.gif" alt="" />
Anyone having experienced a charging bull will know that it maintains a centre line in the moments right up to the explosive attack, when it wounds from either horn by turning its head.
All in all, it is a very natural way of feinting your opponent.
The instabile nature of the posta is clearly that - unstable. You can come in from either direction, or transition to an entirely different posta quite quickly. It is amazingly versatile, despite its outwardly cumbersome appearance. Like Posta di Donna, for example, it is deceptively effective.
In the interim, Bicornio maintains the threat in the high line, to the face of the adversary, without over-extension of the blade.
IMHO, Bicornio is simply another variable Fiore throws into the mix, with which to gain the advantage of surprise in your fight.
I hope this makes sense.
Honourably,
Bob Brooks
British Federation for Historical Swordplay
http://www.bfhs.org