A central idea in the study of Aikido. Each technique employs circuits of neutralization, revolving around the central energy, or hara, of the defender. When combined these circuits create a sphere of protective energy, which is used to deflect and immobilize attackers
These circuits can be characterized into two main groups. First are the closed circuits, typical of techniques known as immobilzations. These techniques involve the extension of aggressive energy into a circuit around the hara, making the attacker the target of his own attack.
The second set of circuits are known as projections. These involve redirecting the attacker's energy around the defender's hara, deflecting the attacker from his target, and sending him on his way.
These circuits of neutralization can be combined in any number of ways, creating the dynamic spere.
On a side note, there is also a book called "Aikido and the Dynamic Sphere, and Illustrated Introduction". This book is an excellent source for technical information on the art of Aikido.