The Four Energies of Tai Chi are:
1. Peng – Ward Off
2. Lu – Roll-Back
3. Gi – Press
4. An – Push
These forces, energies or concepts form the basis for all movement in Tai chi. The outer shape of the movements are not difficult for a beginner to learn, but the internal structure is difficult.
The first energy is Peng, or ward off and it is a long-range, expansive usually upward-moving power.
The second energy is Lu or roll-back. It uses the energy of absorbing and leading. It is also used to coil force.
The third energy, Gi is press, it is usually horizontal straight out and is short-range penetrating power. It is difficult to learn.
The fourth energy, An or push is simple but powerful even for the beginning tai chi student.
Below Mr. Ang corrects and adds some insights.