I think this event largely illustrates something I had noticed before: that although with some incidents you get plenty of warning that you are going to crash, in many cases it happens suddenly. In this case the captain had time to tell the passengers to brace, but not to explain what "brace" means.

So you need to pay attention the first time the safety briefing is given because it may not be repeated. I suspect some of the people who ignore it, don't realise this.