Where does Taichi power come from?

I have learned Traditional Chen style Taichi for around five years at Chenjiagou Taijiquan School (陳家溝太極拳館), founded in Henan (河南省) by Master Chen Zhenglei (陳正雷), one of the Four Great Taichi Masters.

Here is a maxim of Master Chen, it gave me a very important idea in practicing Taichi.

When practicing Taichi, the first thing you need is an accurate understanding of Taichi.
What is Taichi?
Why do you practice this way?
If you don’t understand it, you will lose the direction of where you are practicing.
The next important thing is how to evolve your stiff mind and body into a soft movement through this accurate understanding.
If you can abandon all your stiffness, then you will be able to experience the essence of soft movement called ‘Roujin (柔勁)’.
And if you can do ‘fansong (放松)’ and relax your mind and body further, you can produce fundamental vital energy ‘qi (気)’ in your body.
If you can produce ‘qi’ in your body, you can circulate ‘qi’ throughout your body.
At this stage, the body and movements can be cooperate at a higher level, movements will lead ‘qi’, and ‘qi’ will lead movement.
Guide your ‘qi’ with consciousness and move with ‘qi’.
Every part of your body moves naturally when ‘qi’ comes to it, and it doesn’t move unless ‘qi’ comes to.
When you reach such a higher level, you will be able to grow and to charge your ‘qi’ in true meaning.
Only when the whole body is full of ‘qi’, you can use a certain Taichi power called ‘Gangjing (剛勁)’ freely.

— Chen Zhenglei (陳正雷, One of the Grand Masters of Chen style Taichi)

As an essential idea for practicing Taichi or Bagua, we keep this theory strictly in our daily practices.