Learning Fiore De Liberi

This particular pairing pits a ward suited for thrusting, against a ward suited for breaking the thrust. Posta di Dona Senestra can also make excellent Fendente's, but takes a longer tempo to get the blade around to attack, in this long tempo Finestra can thrust. However, with the blade cranked around the back of the head, di Dona Senestra is also nicely suited to deliver it's own thrust should Finestra takes it's point off line to deliver it's own fendente, mezzana or sottani. In their response to any cut, both wards do the same thing, deliver a thrust before the cut can materialize.
Where they differ is in their responses to the thrust. Both can, and should, respond to a thrust with either Breaking the Thrust, or Exchange of the Point. However, Finestra has some very bad obvious responses, that di Dona Senestra does not have. Finestra is back-weighted, but the sword remains forward. As di Dona moves to attack with a thrust, there is a temptation to simply set the incoming thrust aside, with Finestras point, and wheel the blade into a Fendente. This would tend to keep Finestra's blade inside di Dona's, and therefore gives the appearance of safety.
The first problem with doing this, is that Finestra's blade is trapped on the inside, when the blades Cross at the Tip. The only option here is for di Dona to cut to the other side, and with a traverse to cut Fendente to the other side. If di Dona misses the crossing at the tips, then the two will end up crossed at the middle, again, since Finestra's blade is trapped inside of di Dona's, there is little Finestra can do to stop Finestra from either grabbing the blade, or more likely aiming a fendente at Finestra's hands. Finally, if di Dona misses both the crossing at the tips, and the crossing at the middle plays, di Dona can simply push the exposed Finestra elbow.
As with all the contrary Posta, both these Posta have the exact same options, and responses. What makes this pairing different, is the particularly bad response Finestra has to a thrust. It should also be pointed out that contrary posta are used to see what you're opponent will do. One of the things you can test, especially from di Dona with this pairing, is what your opponent will do when di Dona thrusts, as long as you're prepared if Finestra decides to Exchange, or Break the thrust.
Also, looking at how these two are paired, they are both back weighted with the sword as far back as you can get it. Posta di Dona is cranked around to expose the back to a thrust, while Finestra is also back weighted and inviting the Fendente that di Dona wants to deliver with such power. Since Finestra's blade is withdrawn so far back, Finestra is also guarded against the possibility that di Dona Sinestra can come the narrow plays, as di Dona Sinestra is so good at.