I think so, yes. But I imagine at least part of what happens is people have stop losses in place, which trigger instantly at the opening bell due to the new low

I think so, yes. But I imagine at least part of what happens is people have stop losses in place, which trigger instantly at the opening bell due to the new lower price, plummeting the prices more. Since they would not trigger after hours. But I am actually curious what is happening, where is most of this volume coming from? I can't imagine people had chances to make any decisions in the 1 second between opening and halt. Which I think is the purpose of the pauses, right? Giving humans a chance to make decisions instead of the algorithms?