This drill follows on from the Breaking the thrust drill, and uses the 18th play of the zogho stretto to counter the roverso mezano, which is then countered by the 19-20th plays (soprana tor di spada), which is countered in turn by the 17th play.
The drill works quite differently cross-handed. As a general rule, when dealing with sword blows, mandritto-roverso instructions are to be followed as you see them so regardless of the handedness of the opponent, but when dealing with close combat techniques (elbow pushes, ligadure etc.) such instructions should be followed as they are, so the handedness of the opponent matters more. In this drill the "roverso mezano" into which you counterattack with a mandritto mezano should come towards your mandritto side. When the opponent is holding the sword in his other hand, this works against his mandritto. The exact mechanics of the disarm require some tweaking for the cross-handed play. This is no longer really the 19th and 20th plays of the Getty MS zogho stretto.