An attacker hits you. However, instead of wincing at the pain, you send the energy of the attack back into his arm. While your attacker thought you would be doubling over in pain, he is the one who is hurting. This isn't just something that happens in action movies or video games. This is the power of Tai Chi Chuan Iron Body.
Iron Body training can be divided into two categories - Internal Iron Body and External Iron Body. External Iron Body is predominantly gained through repeatedly striking ones body on a regular basis to take harder and harder hits. While this will develop the body feel that you want, the training is not good for you in the long term because the hits will cause a lot of damage. However, Internal Iron Body training does not have the same kinds of harmful effects.
With proper training, Tai Chi Iron Body can allow the practitioner to feel to an opponent like "steel wrapped in cotton" a famous phrase from the Tai Chi classics. And correct training will in fact make your body feel to an attacker as if there were perhaps a quarter inch of soft material stretched over steel. The body might be soft to the touch, but if an attacker hit it hard, then it could do some serious damage.
This feeling is not restricted to when you are struck. If you were to strike an opponent with Internal Iron Body, they would feel like they had been struck by iron. People who are very skilled can even take the incoming energy of an opponent's strike and send it back to them. Truly skilled practitioners can break an attacker's wrist or arm or even dislocate a shoulder using the attacker's own energy against them.
Developing this ability requires knowledge of a number of other skills and the ability to use those skills in combination. The ability to practice Sung, relaxation. This is a deep kind of relaxation that allows a practitioner's body to remain relaxed even in a combat situation. It also requires Peng, or internal connection as well as various methods of rooting. It also requires the ability to feel and move energy through the body. For example, a practitioner would need be able to feel and move energy through the Lao Gong point, a point in the center of the palm and the Yong Quan or Bubbling Well point in the feet as well as throughout the rest of the body. With this training it is also very useful to be able to feel anywhere inside an attackers body and be able to target specific internal organs at will.
There are other aspects of energy manipulation that a student will need to understand in order to perform these techniques. Practitioners will need to be able to work with earth energy and heaven energy both on their own and together. They will also need to correctly practice body marrow washing as well.
Though Internal Iron Body takes some serious training to achieve, it can be a powerful tool, and it will make your body much stronger and healthier over time.