[lancefisher]

I would guess the wind blows into the slit, driving just one side of the paddle.

[Jestar342]

Just watching the video shows how they work just the same as a water wheel does, except "on their side". The wall(s) erected around them are shielding one half of the windmill, thus there is wind flowing onto one side of the windmill only.

[mattlondon]

Hmm yes of course :) So in the videos we are seeing the "back" of the windmills.