Blair, I think I know what's confusing you about Trevor's very good analogy. The Ferrari and dump truck speeds are not always constant relative to each other and neither is the length of the towbar (phase angle is a function of frequency - and amplitude too!). Does this help? LOL! It's all so complex not even the hand of God can make it right.
Those VHS tapes of hers have to go. They're really messing with the spectrum above 1.28KHz.