There are many different "energies" in Tai Chi and the internal arts. Cai energy is also called "Pluck." It is a sudden pulling action that can take your opponent off-balance.
Pluck can be done in a subtle way in push hands but in real self-defense, when your life can be in danger, it is not subtle and not especially pretty -- but it is definitely effective and it can be used against all kinds of attacks.
Here is how to begin practicing this particular method of Tai Chi Pluck energy. Have a partner attack you without warning. Your first goal is to cover and block the attack. Instinctively, you should drop your weight and avoid the lifting of the body. This takes a lot of practice and presence of mind. In fact, it is a very good idea simply to react to an initial attack by practicing -- over and over -- the dropping and covering technique to protect yourself in the event of a surprise attack.
Despite what some people claim, not all attacks are surprise attacks. Very often, guys will go into their "monkey dance," and give you some warning they are about to strike. Sometimes, the initial action will be to shove you back. Either way, once your attacker invades your space, covering and blocking is an appropriate response.
Next, as quickly as possible, grab the attacking hand (or foot if it is a kick) and give it a sudden, hard jerk downward. This will take your opponent off-balance, usually in a big way.
Internal body mechanics are required. To pull with sufficient force, you must be connected from the ground of the front foot through the hands. As you pluck, the whole-body connection allows the ground to work with the arms and the Dan T'ien rotation and you close powerfully into the kua (my right kua in the photos) as you jerk the opponent down.
Be careful when doing pluck in this way with a partner. You can literally give a partner whiplash by jerking quickly and powerfully. So make sure your partner is ready and take precautions that you don't cause injury.
This is an extremely effective technique for real self-defense. Remember, in the internal arts, your goals are to uproot and unbalance your opponent. You do that by neutralizing the incoming force and countering with good internal body mechanics. Tai Chi is not always gentle. It is a brutal fighting art. Pluck Energy is an excellent self-defense technique. Try it against all types of attacks. Pad up and use it in sparring. Try it against combination attacks. Any time you can grab your opponent, pluck is an effective way of disorienting him to set up your counter-attack.
The video for this lesson is in the Internal Strength section of my website at www.internalfightingarts.com.