This is Posta Coda Longa (Long Tail Guard) that stetches down to the ground behind you. She can thrust and also cover in front and wound. And if it passes forward and strikes with a downward cut, it enters the narrow play without fail. This guard is good to wait in to quickly change to other guards.
In this posta, the sword is held low and behind you. You will need to turn your hips slightly more to the front than usual. Keep your left elbow resting at your hip, with the forearm crossing your centre of gravity. This anchors the blade directly to your core. It is this connection to your centre that puts the posta in the stable (stabile) category. Extend the blade behind you with the true edge down. The length of the sword is hidden from your opponent behind your arms.
By keeping the left elbow still and pivoting the sword from there as you step, it is possible to deliver a cut which has not only the momentum of a large swing, but which is also delivered with surprising speed. This is a good method to sweep away your opponents defences or attack directly as you rapidly close to the narrow play.
You can see other examples of Posta Coda Longa in the following.