Ki, the green bar, is how you can perform one of your attacks or roll during combat in Nioh 2. You have a limited amount of ki at your disposal, and the more you use it, the more you will get. you empty. When empty, there is nothing you can do to defend against an enemy attack until it recharges. However, if you're quick enough, you can regain some of your stamina lost during a fight and not have to wait as long for it to return.
When you perform an attack on an enemy, your green bar at the top left of your screen will gradually descend, indicating how much stamina you have left. Although it recharges slowly during combat and you don't do anything, it's a slow process. It's way too slow if you want to take on tougher enemies that are blazingly fast. To quickly restore your ki, watch for the green bar that turns red for your lost stamina, then a white bar should pull up the red bar. When it reaches the peak side of the red bar, press the R1 button, and you will find that many ki.
You have to be quick. If you are too late, you will lose this ki and have to wait for it to recharge. If you do it too early, you get some of your ki back, but not all of it. But doing it too early is better than too late. By learning the timing of this ki recharge, you can optimize your fight time to take on tougher enemies by dealing a large amount of damage to them and still being able to defend yourself or move away.
Regaining your stamina goes hand in hand with making sure you fully understand the type of attacks your weapon has when you change positions. After regaining your stamina, try changing your stance to start making new attacks, pushing your enemy back even further.