In our babies, we commonly see this when mom was a poly-sub abuser; once you see it, it's hard to forget! Try this: Take your arm and hold it out in front of you flexed at the elbow. Tense all your arm muscles very tightly and then 'jiggle' your arm very quickly- like muscle tremors? Like you've been drinking too much coffee?
That's jittery. Often you'll see this in a newborn who has been exposed to crack or cocaine (at least, this is what we see a LOT...we get a lot of cocaine abusers in our area). Another way to demonstrate this is to tense all the muscles in your neck and jaw until your head starts to sort of vibrate and shake? That's jittery. In a baby, you'll usually see that they startle easily (sometimes for no reason at all, ie, no sound or stimuli; just happens out of the blue or when they get extremely agitated) and begin flailing their arms and/or legs, and then the flailing starts to a sort of shimmy-like muscle vibration. It's really not hard to spot when you see it, because normally babies don't do this. ;>)