Thank you! That link is much clearer than the article. One thing wasn't in it that I found cleared this up a lot, though: with a normal sail, wind that gets c

Thank you! That link is much clearer than the article. One thing wasn't in it that I found cleared this up a lot, though: with a normal sail, wind that gets caught still moves forward relative to the ground, just slower. With this thing, it winds up going backwards. That supplies the extra needed momentum.