This is Posta Breve (Short Guard) that wants a long sword and is a malicious guard, but has no stability. Also always move and see if you can enter with a thrust and step against the opponent. It is more appropriate to use this guard with armour than without armour.
Rest your left forearm against your body, with your hand resting on your centre of gravity. Adjust your right hand so that the point of the sword is aimed directly at your opponents eyes. Relax your arms and keep your elbows in.
The blade runs straight along the centreline to your opponent. Pointing the blade directly at your opponents face gives a strongly foreshortened view. To a certain extent, this hides the action of the blade.
Although you cannot make effective cuts from here, posta breve is very good for making probing thrusts against your opponent.
Fiore confusingly classes this as a stable guard, and then immediately states it has no stability. As you can see, the sword is attacked directly to the hips, providing it with a great deal of strength, especially in the vertical plane. This is what causes it to be classified as stable. Fiores advice to keep moving when in this guard, as well as his claim that it is better to use in armour, suggests that it is limited in its capability and risky to wait in. This could be what he means when he states it has no stability.
In the ‘one is like the other’ section, Fiore tells us that each posta is its own counter, with the exception of those guards with the point on the centreline, such as this one. In this case the longer sword holds the advantage against its shorter counterpart due to its increased reach.