Tai chi is the use of relaxed leverage and a detailed understanding of anatomy and physics to defend yourself from attack. The reason why proficiency takes so long, is entirely due to the incredibly detailed instruction required to make you stand and move in the right manner.
So, like a tree is strong due to it's deep roots, a Tai Chi practitioner 'borrows' the firmness of the ground, and uses it as a basis for self defence and expressing power. This connection with the ground is an attribute called, "Rooting".
To get your rooting correct, you need to have a good understanding of the interplay between yin and yang in the body. At an elementary level, this is simply the distribution of weight between the feet. Yang being substantial and yin being insubstantial. Rooting develops when you can always keep one leg more substantial than the other. Also having the body relaxed and alert in the right places.
Another attribute of good rooting, is an upright posture of the spine, and to have all components of the body in alignment - The knee, hip, shoulder and big toe should all be in alignment. The body should be supported on all sides, with no overbalancing.
Training your rooting involves 3 different techniques:
Standing in single positions for long periods of time, gives you awareness of unnecessary tension. A good teacher will adjust your alignment so there is minimum stress, and maximum rooting and power available.
2. Solo form and movement
The Tai Chi forms are excellent for ensuring you are rooted. Chen Tai Chi also has the silk reeling exercise sets, which are excellent for making the practitioner aware of the path that force takes through the body.
3. Pairs work - pushing hands
Pushing hands is the bridge between the form work, and being able to defend yourself in a real situation. There are exercises particularly suited to testing each other's rooting. While these exercises have become competition events, they are fundamentally about people working together to discover each other's weaknesses and how to mitigate them.
Finally, one must always guard against being too rooted. If you are rooted to excess, you will not be mobile enough to evade attack..... But that's a subject for another day :)