Thank you all for the explanations,
I was pretty close to grasping this, but the T-Valve explanation makes this a lot easier to grasp. Rather than thinking of only one way for the electricity to flow, I need to realise that it can also flow to ground. As per high school science, electricity chooses the path of least resistance.
Am I right in assuming that since V=IR, as we increase the volume, the tube gets more current and therefore the volume increases? Thus, could the "level of signal" in Doc's post be replaced with current? Also, with the attentuator on anything other than a zero setting, does some of the electricity actually flow to ground, or is it current and resistance merely changed (again V=IR as per the above) and all of it flows to the tube?
Damn, I am now realising the extent of my ignorance when it comes to electricity...