Sometimes I have no words to say other than if you want to master Tai Chi the easiest thing is just to try it out. If you don’t get it the first time have some faith and try again and again. Some things are not easy to explain and even if explained would still be difficult to understand.
For example in a previous post I mentioned about this particular shaking ruler method from the Tai Chi Ruler style. I told my student that the principle of fajing here is that you shake the ruler to vibrate the dan tian.
Once the dan tian is awakened and shaking it should take over the shaking motion used initially by the arms. You can say that when you started the external shakes the internal. Now that the internal is there the case becomes the internal shakes the external.
As you keep shaking the ruler there are more interesting effects that can be observed. One of them is how the shaking of the dan tian can produce fast and powerful fajing like movements that seems out of your control. The arms will shake violently for some time before it slows down and stops.
Over the years, the practice will allow you to internalize the shaking. Now even if you want to shake your arms violently like previously you won’t be able to do it. At this point you have movement within stillness. If you test your fajing now your partner will feel a strong vibrating force coming from your arms to throw him off balance.
How to explain this scientifically? I have no idea other than the Tai Chi Ruler shaking has somehow enabled the body to release the force in a rapid, short span sinuous wave-like manner that approximates the feeling of being shaken suddenly and violently felt by the partner.
P.S.- I tried to find a video on Youtube that shows this ruler shaking exercise but cannot find one. I came across a number of clips on Tai Chi Ruler basic circling exercise and they look more like normal physical exercise whilst holding a gadget rather than use the gadget (the ruler) to engage the intention to train the body’s internal force. This is why the Tai Chi Ruler is also known as Steady Intention Needle. In this case the word “needle” means indicator. Thus, it is a gadget that indicates to you how to train your intention to be usable by focusing it (i.e. keeping it steady).