The Chinese did not invent Yin & Yang, it is the oldest thing in the Universe. Those words are just what the Chinese called 'natural opposites'. One cannot exist without the other and both cannot exist if they are not in a constant state of change into each other and back again.
Inner power has little to do with physical power and how we use our muscles. It's just that the method we use to gain 'Internal Power' lends itself to a better way of fighting using natural breathing and movement. However, you cannot summon up some supernatural power to defeat an enemy! You can simply use a martial system based upon natural movement, which is more economical, more direct, using deadly and explosive fighting methods. So most people mistakenly think that when for instance they take up Taijiquan, that they are developing some kind of Internal Power that will help them in a fighting situation. This is untrue. What the Internal Martial Arts do give us, is indeed the most deadly fighting systems ever invented, based upon the human anatomy taking in its limitations and its advantages.
The Inner Power that we are said to get by practicing an internal martial system such as Taijiquan or Baguazhang gives us a much more subtle and potent power; not for fighting a physical attacker, but for fighting off disease by keeping us in a constant state of balance and to enable us to cause 'things' to happen in our lives! This may sound mystical but it is quite a natural thing. The human brain is tremendously powerful with the ability to make us ill, make us well, make us happy or sad, and enable us to gather wealth or to change our circumstances in some way. And this is the Internal Power that we gain by doing for instance Taijiquan. But by doing Taijiquan in the manner it was invented, lends itself to an excellent way of self defence as an off-shoot of training to gain inner power.
If one has in mind that he or she is going to train in Taijiquan to become Superman, he will never become that. In fact he or she will not get very far in Taijiquan with this thought in mind. Or if someone takes up Taijiquan to be able to throw someone without touching them, they will fail! How stupid, why would God bother wit such a small thing, why do we wish to do this anyway? The human body does not have the capability to become Superman in any way. However, if you take up Taijiquan (or any internal system) with the thought of becoming a good martial artist being able to defend yourself and family, or that you wish to become healthy and to be able to make things happen in your life for the better, then given the correct instruction and diligence, you will succeed. And along the way, you just might pick up one of the best self defence systems ever invented.
However, the real reason for doing Taijiquan will eventually far outweigh the childish notion of martial arts in that it will take you to areas that you only dreamed of; areas that you thought only happened in comic books or science fiction movies. You will eventually have the power of your brain at your fingertips! You will find that you will be able to simply 'will' something to happen and it will. This may happen through prayer or through meditation etc., or a quiet word with God or one of your 'Guides' in life.
However, you must do some things and give up some things, make sacrifices in order for true Yin and Yang to kick in. If you have Yin, there MUST be Yang and visa-versa. So if you wish to gain something such as inner power or to have something happen in your life, something great, then you must be willing to give up something in order to 'Invest In Loss'.
So apart from the physical training of an internal martial art, you must also make sacrifices in order for there to be an opposite 'Yang' happen in your life. In other words, once you have the Inner Power, you must have some way of accessing it to make it work for you. And we access it by loss.
We as humans are relatively comfortable; we do not like changes in our lives. We do not like our daily routines to be disrupted. With this attitude, Inner Power will stay forever immobile. We become dependent upon our daily routines like eating, having sex, drinking social drinks such as coffee or tea, eating and drinking when we do not need it etc. Smoking or drinking other poisons such as alcohol and other drugs. No matter how much Taijiquan you do, even if you are one of the lucky few who has a great teacher and style, you will never realise Inner Power. Doing drugs or alcohol or smoking, you will never even build up any Inner Power to begin with let alone be able to use it to make changes.
What did all of the greatest leaders of all time do in order to create change? Men like Ghandi, Jesus, Mohammed, Krishna, Buddha would invest in loss in order to tap into the power of their mind. They would give up one of the most difficult things for human beings to give up, FOOD! And Pleasure SEX. Now, I know that I have probably lost 99% of my male readers and 75% of females. However, if you are serious about gaining Internal Power, then you will have to at least go on a food and sex fast, not forever, but for a period of time, enough to allow your body and mind to become unclogged.
Even the humble 36 hour fast is far too drastic for most modern Westerner people who have been eating junk food most of their lives and the mind shock is just too great so they give up long before only 36 hours has been reached. They give excuses such as "I have to work and I need energy", or "I feel really ill" etc. So, you do it when you do not have to work hard perhaps at the weekend. Even this short fast will have an effect upon your Inner Power, allowing for the beginning of your change to happen. The more you do your Taijiquan of course will also have an effect in that it will happen sooner, the longer you have been practicing.
Animal fats and indeed all animal products for consumption will block your Inner Power. Sugar and other refined foods will also, dairy products including cheese will definitely block your Inner Power from being developed. The more your body has to work and use energy (Qi) for digestion, the less you will have for the big picture of changing your life or realising your dream.
I have personal experience with this kind of Internal Power and its gathering. I cannot begin to write about the 'strange' things that have happened just after a fast and even months after. Even when I go back to my normal eating routine (which is pretty bland anyway), the short fast keeps things happening for some time after. As I have always said, you must be doing a great form of Internal Martial Art, not some watered down version, flapping the arms in the breeze, dancing etc. And you must be in a state of complete balance when you perform your Taijiquan form for instance. And you cannot get that without one of the major original forms of Taijiquan, not some shortened version. If you have a good form of Taijiquan, one that will get your motor started as far as Internal Power is concerned, then once it is gathered, it's just a matter of putting it into gear by investing in loss with a short 36 hour fast, only taking pure water from one evening to the morning of 36 hours later. You will feel ill and many weird things will happen in your mind. But once attained, the short fast will become a part of your life and will not be so difficult.