Agreed with Craig. Ki is a measureable, useable phenomena; what exactly it is
is anyone's guess.
But whatever it is; it ain't what they do in 'Dragonball Z'.
My personal take on it is it's two things combined: 1) the focussing of intent or will along a specific path (i.e. 'extending ki to the wall'). 2) the physiological effect of a psychological state (i.e. Unbendable Arm - by visualizing ki extending out the arm, one uses only the muscles one needs to hold it steady; rather then whole muscle groups acting against one another).
Right or wrong; I dunno but I'm gonna wait for shodan before I start practicing throwing ki-balls.