Additions are where the true talent of the game lies. Being able to complete them can make or break your game. They are simply sequences of attacks that each character (except Shana and Miranda) uses, and are completed by pressing X at the right time. The "right time" would include when the two blue boxes meet or directly after the button on the right side of the screen goes down.
When an enemy counterattacks, you must press O instead of X to complete the move. The enemy will appear to attack you, flash red, and the boxes will also turn red. Simply press O when you would have normally pressed X and you will continue to attack. However, if you fail, your character will get blown back, taking damage.