Well, for me, the day I really understood the BEAM is the day Elixir blown my mind. And two talks helped to understand that.
In the first one @josevalim takes some time to explain the concepts behind Elixir (which are pretty much the concepts behind the BEAM), and it blown my mind because it’s so much simpler and more organized than OO that I cannot call it other way than a masterpiece!
And in the second one, @sasajuric explains in a very practical way what is the real power of the BEAM: its concurrency model, the runtime guarantees it gives you, the price you pay to have them, and why this price is fair. 
I know one can argue that it was not Elixir that blown my mind then, but the thing is: other BEAM languages never got me interested enough since I felt too productive with Ruby compared to them, and Elixir changed that! The efforts on making it easy to learn and productive really paid out!
