Carrying the sword like this against the spear is very good to beat the lance when riding on the right side of the player. This guard is good against all other hand held weapons, such as pollaxe, staff, sword, etcetera.
The master here carries the sword in Posta di Donna la Sinistra. Because it is a single handed sword, and also because you have to sit squarely on your horse, it cannot turn so far around you as when making the same posta with a sword in two hands. The torque you lose from the reduced body mechanics, however, is more than compensated by the momentum of the horse.
From here, you can cut over your horses head on a downward diagonal to the right, beating aside your opponents weapon. This is an effective defence not only against mounted opponents, but also as a general defence against any opponent with any hand held weapon.
In addition to beating your opponents attacks down, you can also make powerful cuts or thrusts of your own from this position. You should also be able to enter any of the plays of the 8th master.