A counter-posture is a positioning of the body and sword so that there is no straight line your opponent’s point to your body. This way you can be assured that even if you don’t move he cannot attack you without first repositioning his sword. This of course forces him into a longer tempo, giving you plenty of time to parry.
In order for this to work your sword must be positioned such that it is stronger than the opponent’s (it’s no good if he can just blow through it). Don’t rely on a dagger or buckler, your sword must stronger even if you have a defensive item. And if you can subtle about it all the better.
Your opponent will be trying to form his own counter-postures, so be prepared to mutate yours as needed. Being able to draw upon lots of counter-postures is vital.
The game of counter-postures should be played just barely outside measure. If your opponent steps back while changing his position, step forward as you counter to regain the original measure and advantage.
If you are already within long measure (misura larga) then consider attacking when the opponent changes position. If that is not viable, then instead form the new counter-posture by moving the sword alone. Don’t move the feet, your tempo will be way too long. Besides, you need to leave open the option to step back if attacked during your mutation.
Form your counter-postures in a slow and controlled manner. If your opponent interrupts your mutation you will be able to abort it and instead parry and attack. Rushing from guard to guard is not to your advantage.
You need to form your counter-posture before gaining measure. If you find yourself shut out of line by your opponent’s counter-posture, then step back and break measure. Don’t continue advancing from a disadvantageous position.