Peter, I'll be anxious to see the metaphors up with which folks come. (Apologies to Sir. Winston) I can't think of a single one. All the rules, like keeping your thumb squarely on the back of the neck, seem to go out the window in folk and popular styles of playing.
When I play finger style I lead with my thumb and most of my thought process actually centers around the bass line rather than the syncopated melody being played independently by my index finger. Often this sense of rhythm is carried more by very slight movement of my forearm than by any concentrated movement of the thumb itself. It seems to take more thought, while playing, to keep the rhythm steady and unbroken than it does to play the melody notes.
One thing I'll never understand is how rock performers are able hang their guitars down around their knees and still form chords. I tend to adjust my guitar strap so the neck is nearly in a classical (seated) position while I'm playing. I don't have to go for what looks cool on MTV, I just try to position the neck so I can play.