Meltdown is like having a strong emotion inside and you have no idea what to do to get rid of it. It forces you to cry, yell, jump, hit stuffs. You feel overwhelmed by it. You know it is stupid and you shouldn't feel that way but your can't control it. You have some control over your actions (you can hold them a few mins and escape somewhere where you are alone, you can decide not to hit things and just hit the air instead etc. but thats it) but not the emotion itself, you have to let it go, somehow. If you don't do it fast enough the emotion might destroy something inside you and the avoided meltdown changes into a shutdown. You get "blind" and "deaf" to the outside world for a while because the emotion was too strong for your senses. Shutdown is like temporary blindness after looking at a strong light.