When testing a student using a simple shoulder push I can feel when she goes from active muscular resistance to being moved to a state (we call correct feeling) where the force I am applying simply has no effect on her and she is able to stand stock still with little to no effort. The state of correct feeling is achieved when the student learns how to coordinate mind and body. Ki is manifest when correct feeling is achieved as a result of coordinating mind and body. Extend Ki is shorthand for the instruction "coordinate mind and body in order to achieve correct feeling." Once the student learns how she feels when performing this simple test she can then replicate that same feeling at will with other tests or when practicing technique.
That's fine, but I'm with Krystal: it sounds like mechanics to me (directed by the mind, but be sure, but aren't all volitional movements?). I'd like to thank you for clarifying your terms, though. I get the feeling that when most people use the term "ki", they don't mean what you mean...but I don't have a dog in the "what is ki" hunt at all, at all.