In Pearls on a String Theory, the body is set up like a complex beaded necklace or chain. It is easiest to imagine the individual joints as the joining of two of these pearls. Movement is generated from the joints surrounding the hips and moves out from there.
When applied to the arms alone, which is the normal starting point for most beginners, the process is similar to the action of a whip. For example in the standard brush knee left (where the left hand performs a redirect that would appear to be a low block to an external martial artist and the right hand performs a shoulder height palm redirect or strike) the right hand is first raised above the head palm down.
If we apply the theory to just isolating the arm we see that the arm moves in a "S" or whip like movement, the shoulder releases dropping the upper arm followed by the forearm this bends the wrist so that the palm which was parallel to the ground to face forward. If all the joints are moving in this sequence, they will all come to rest at once causing the palm to in the simplest version of this movement to generate an outgoing force that is forward and upward (using the wrist as the fulcrum for the movement).
When this movement is applied in self-defense, it also includes a forward shift of weight and a circular movement of the body. So in the given example the full power of the arm, and the kinetic potential of its drop, is channeled into a single lifting motion of perhaps a few centimeters lifting the opponent as the rest of the force is applied by the body.
Additionally if these actions are preformed fast enough, tissue elasticity can come into play, which can create additional power. (I will discuss this in a separate entry. This is the groundwork for the internal martial arts punching style and is most prevalent in Hsing Yi forms.
For those familiar with the body's skeletal and muscle systems, the enormous amounts of movable joints it is clear just how much power could be generated just by a simple "shaking movement" of the body. (I will stress here, as I will throughout these entries that while I am documenting the theory I make no claim of mastery or proficiency beyond the training methods for these techniques.)
For those who are looking at the therapeutic side of the internal arts, training in this theory with the understanding that each joint should maintain some give. Within medium frame Tai Chi Chuan training we follow a seventy percent rule for joint flexing... an elbow is never completely locked open or completely flexed so that the upper and lower arm meet. The same theory can be applied to each of the joints of the body including those of the spine.
This combined with the bone to soft tissue bows (yet another entry that will be made) produce a postural position that is both extremely sound and flexible at the same time.
This should only be viewed as a simple introduction to these concepts as with most of the internal martial and health arts extensive person-to-person instruction, and individual practice is required to master any of these concepts.

Author's Bio: 

Bruce Hutchinson has studied Martial Arts for 25 years. Currently he instructs a Traditional Tai Chi Chuan and Qi Gong program as the Head of the Chinese Martial Arts Group a division of Young's Tae Kwon Do in Bellefonte, Pa and Tai Chi For Wellness through the faculty and student fitness facilities on Penn State University's main campus.

His programs have been offered by hospitals, insurance wellness programs, YMCA/YWCA, and assisted living facilities throughout Pennsylvania.