- Completely regenerating health (not just the non-red part)
To be clear, there is an energy bar and then there is its "container". As you get hit, you lose energy and the container shrinks as well, but un-proportionally so (less than the amount of energy lost).
Now, as far as I can tell, the Ninpo puts a pre-defined amount of energy back into the container. If the container is bigger than the pre-defined amount of energy you get from the Ninpo it doesn't get completely filled. If it's smaller, tho, it gets extended by the amount needed to host the pre-defined amount of energy, meaning that by casting a Ninpo when you are full of energy, the container grows (until it eventually reaches its limit).
So basically it's good practice to unleash a Ninpo at the end of a fight even tho there is only one guy left and you have your container full because you'll enter the next fight with a bigger container/energy bar.
All of the above changes depending on difficulty I think (container fills/grows more/less etc).