I agree I've never interpreted it to mean throwing my head back, I've always done it in a controlled manner.
I will say though that ocasionally I do find I wedge my hands in a bit on each side under my glutes in the 2nd rite, so does my daughter.
Also in the third both me and my daugther seem to prefer to curl our feet up onto our toes instead of putting the tops of our feet flat on the floor, though I do switch from both. Has anyone else experienced these things?
But in general I wouldn't modify them too much because they seem to work well the way they are.