@Wordist, it's simple conservation of energy, the inverse square law of gravitational attraction, and the fact that seabed friction already causes the ocean's tidal bulge to lag slightly behind the moon, and therefore slow it up. A tidal barrage holds the bulge back some more, so it will slow the moon up more. This was all covered in first year at university, some of it in Physics and the rest in Astronomy. We could all see it then, so I'm sorry, I can't help any more if you can't see it now.