I like turtles.
Not really. I gave you the context in which the phrase was used in post #29.
I believe that in the novelization of "Star Trek: The Motion Picture", Gene Roddenberry himself described the impulse drive as being a fusion rocket engine. Rather than hydrogen and oxygen burning chemically, hydrogen alone was caused to fuse and then dumped out the back to provide thrust. Similar to a conventional rocket.
I'm going purely by memory though and do not have a copy of the novel handy.