It was definitely a metaphor that I picked up somewhere, though I don't recall where. Definitely stuck with me though, primarily as a way of highlighting how one type of work affects another, but has diminishing returns as you move away from that type of effort.
To girlfriend: I think you've aptly picked up on an important point of the metaphor, as well: That when you're new to it, no matter where on the tube you squeeze, the tube is going to produce. Once you've been at it for years, it really starts to matter where you squeeze, for how long, and how quickly you move from the base of the tube and back.