He screwed the pooch on this one. He clearly only left the fan on for two seconds because he didn't want to explain why his boat started moving when he left it on longer, whether that movement was spinning or moving forward. The sail isn't a perfect wall that negates the forces of the fan and so some of the air will blow back or to the sides off of it and make the boat do something.
There's lots of other great info in the video, but he used this sail thing as click bait and is showing he cares more about selling his products now than really teaching. Sad.