The Double Palm Change is the second of Ba Gua Zhang's eight mother palm changes and it contains the foundation of all the yin (soft) techniques of Ba Gua Zhang. The I Ching or book of change on which Ba Gua Zhang is based states that "the receptive (yin) completes the creative (yang)" and as such the Double Palm Change builds on and completes the Single Palm Change.
Where the Single Palm Change begins with the manifestation of yang energy, the double begins with the manifestation of yin energy. Yin energy is vital for developing and activating the natural healing mechanisms of the body, it also balances the mind preventing excessive yang emotions.
In terms of the physical health of the body the Double Palm Change is similar to the method of Taijiquan it emphasizes deep internal twisting and spiraling and a soft inward stretch rather than a yang expansion. This soft internal twisting massages and opens the body's internal organs and soft tissues. Where Ba Gua Zhang's yin techniques are slightly different than Taiji's is that they emphasize dispersal of energy rather than absorption. This is primarily due to the fact that the feet are generally moving in Ba Gua Zhang and generally planted in Taiji.
For martial artists the techniques of the Double Palm Change begin to expand to multiple attackers. It is Ba Gua Zhang's ability to fight multiple opponents at the same time that sets it apart from most other martial arts. Bruce wrote about his teacher Bai Hua who had first hand experience using Ba Gua Zhang against multiple attackers during the Cultural Revolution.
The rapid changes of direction and vertical dropping and rising within the Double Palm can greatly expand on the martial techniques developed in the the single palm change and really develop strong healthy legs. The addition of low stances and multidirectional strikes vastly increases the martial arsenal of any fighter or push hands player.
For students interested purely in the meditative aspects of Ba Gua Zhang the double palm begins the process of working with the etheric or "chi" body which directly effects many of the "lower" emotions.
For students who have learned the Single Palm Change, learning the Double Palm Change extremely valuable in that it will expose whatever gaps may exist with in the single palm and with practice begin to fill in those gaps. Only when the energies of yin and yang can smoothly mix and integrate can one begin to work on the manifestation of the other energies that exist within the remain six palm changes.