Translation
This is Posta Bicorno (Two Horned Guard) which is locked so the point is always in the middle of the line. And what I can do with Posta Longa, I can do with this. And similarly I say for Posta di Fenestra and Posta Frontale.
Interpretation
Posta di Bicorno is surely the most misunderstood posta in all of armizare. The description of the text is not especially clear, the posta only appears once, and the pictures from different manuscripts show different grips.
The two horns in the name refer to a two horned anvil. Armourers will be familiar with these pieces of equipment. You can see this concept reflected in the text where Fiore tells us that the posta is locked in the centreline. This posta will not be beaten aside regardless of how hard you hit it.
Having said that, it is also classed as an unstable (instabile) posta. The sword is separated from your core, and is held in place by the strength of your arms. You can transition in and out of this posta, but should not rest there.
The biggest point of contention with this posta is the grip. The Getty, the Florius and the Morgan manuscripts show the left hand with the thumb facing back, or even cupped over the swords pommel. This feels completely counter intuitive, but it will make for an incredibly solid blade that is very useful in the exchange.
Folio 24 v. b 
Posta di bicorno – Florius 
Posta di Bicorno – Morgan
From a centreline posta, such as posta frontale, roll your right hand inward 90 degrees so that the true edge of the blade moves from the bottom to the left. As you do so, release the grip with your left hand. Maintaining contact with your palm, roll your hand back 180 degrees and then take hold of the handle again with your thumb towards the pommel. Keep your elbows in tight, your hands close to your chest, and your forearms braced against each other.
This gives a very strong line which will redirect a thrust from the bind. Although you will not be able to extend your arms, you can use your legs to make up distance, stepping into your opponent. Your sword will push through your opponents defences.
Alternatively, you can keep the thumb forward, as drawn in the Pisani Dossi. The movement of the right hand is the same as described above. With the left hand, simply loosen your grip and allow the handle to slide inside it. Tighten your hand again when the sword is in place. Again, you end up locking the forearms together, which holds the point in the centreline.