Well, here it is. Behold, the extended metaphor:

The above is a high-falutin' mixer board at Smart Studios, where JoyDad's band did the mixing work on their latest CD. (Smart Studios was founded by Butch Vig of Garbage, and has done work for Nirvana & Smashing Pumpkins as well).
But a sound board or light board would work too, anything with a bunch of flip switches and fader/dimmer switches and a variety of knobs.
Here's the thing. Sometimes with Joy, attributes or abilities or favorites change suddenly, like the flip of a two-position switch, as happened with the swinging. Sometimes we get a dimmer switch, like a strange fading of language that has happened in the last couple of weeks (she's basically using no words now). Meanwhile another dimmer switch for experimental vocalizations has been slowly turning on. The biting, fortunately, has faded off, and is now reduced to a rather cute occasional nibbling of toes. Expressing frustration by throwing things, meanwhile, has faded on.
When a whole bunch of switches slam to off all at once (or fade to off fairly rapidly), we call it a regression. But more often, faders are going in different directions, and we don't quite know if they're related, or what might be causing what.
I was talking with JoyDad about this and asked if I could use his photo, and he told me another really cool aspect about the board at Smart: the switches are motorized and you can actually see them moving as a recording plays. Cool, and a little spooky.
The fact that Joy's switches do go both ways feels spooky to me in general. Neurotypical kids, their switches don't do this as much, at least when it comes to learning. Likes and desires are a different matter, but once a kid learns words, you expect the words to be there. Once she can stack blocks, you don't figure you're going to find yourself teaching it all over again.

It's just like playing a huge musical instrument, and your hands never stop moving on the dimmers.
So now my reverent image of omnipotent fingers on Joy's dimmer switches is hopelessly conflated with a mental picture of the young John Elder Robison working the lightboard in a huge arena... Sometimes metaphors extend in really wild directions!
Does the sound-board / light-board / mixer-board metaphor work for anyone else?