As I always say, DD1 doesn't have an ASD dx, but is similar in many ways.
Meltdowns involve (usually) a period of getting completely hyped up, doing more and more unacceptable things, laughing hysterically, etc. Then moves on to complete emotional breakdown, hysterical crying, sobbing, hitting, pinching, etc.
She isn't strong enough to do any real damage. Her hands are weak and her grasp is weak because her fingers curl. But her Piedro boots are HARD.
The only thing that helps is to firmly cradle her in our arms pulling her knees towards her chest and sing 'Rock a bye baby'.
The worst one was around 18 months ago, when she went into complete meltdown half-way home from preschool. I was heavily pregnant with DD3, had DD2 in the buggy and DD1 refused to walk at all, hysterical, etc. I eventually had to sit on the pavement, phone my Mum, and cradle DD1, singing rock-a-bye-baby until my Mum came to take us home.