I saw that same graph on Khan's FAQ today. He was explaining that if there's a lull in the video, then he works to make it more interesting at that point, until the lull goes away.
How do you get that data?
Youtube gives it to you for free but you have to have enough views. It's under "Insights" on the video and then "Hotspots".
We're working on a product that will be able to give you this data regardless of number of views. Ping me if you'd like more info or an early access account!