Peng (pronounced pung) is the first technique in most Tai Chi forms and is the first technique in "grasp sparrows tail" the beginning sequence of the form that contains all the principles of Tai Chi. In the form the sequence is the constructive cycle and in pushing hands it is the destructive.
Peng pushing is to evelop "Peng Jing" the line of energy from the feet to the hands which can then be used to develop "Fa Geng" (often called "Fa Jing") which can be translated as "vibrating warrior energy", where you use the push of the opponent to load the spring into your body and then send it back to him with a shock causing him to recoil from you.