Okay, I see it now. You have to have them grouped by conversation. I see it in emails sent from a regular computer so it's not an iPhone thing. What it seems to do is to expand all of the conversation into the one email when you click see more, then it's like viewing the last email in a conversation when group by conversation is not selected.
Well, you know, I occasionally, click button here and there, on and off, wondering what they do… And then I forget about it. Still, with all the back and forth emails at my job, I thought I would have noticed it before. Pretty cool eye candy though!