The Envisioned Behavior of a system is the grand idea of the functioning of the system. It is for any large system ill-understood. Through describing by implementing it, we start to more fully realize the envisioned behavior. During this implementation process however, the envisioned behavior grows because more of its behavior is thought about by the developer. This is the iteration and learning process.
There is a very large cliff currently in going from Envisioned Behavior to implementation. Chasm is about creating simpler steps by decomposing the problem and increasing the detail of the implementation. This is done by splitting the Envisioned Behavior into smaller Envisioned Behaviors which are responsible for a complete subset of the behavior and using more abstract implementations.
As the system progresses to implementation, the correctness decreases.