It was pretty aggro. You'd open up Reddit, your envelope would have a three-digit number next to it, and if you hovered over anything you saw, you joined the party. Whole site went down for about 18 hours.
It was one of those examples of precocious kids going "hmm - what happens when I do this?" and what happens is AWS gets dragged down by a substantial percentage because Reddit's that much of a hog. Dude did a Q&A about three days later (this was a year or two before IAmA existed) but it's lost to time.