The interesting part is not that they can briefly go faster than the wind, but rather that they can keep this speed up. It's easy to briefly go faster than the wind: use some gears to connect a windmill to a heavy piece of metal rotating on the end of a long pole. Assuming a strong wind, low-friction gears, and that you've chosen the gears such that the metal rotates faster than the windmill, the machinery will soon be spinning quite rapidly. Now, just put a hapless pig in the way of the rotating piece of metal, and you suddenly have a slightly-dented pig flying at greater-than-wind speed. ("With sufficient thrust, pigs fly just fine", although this will definitely annoy the pig.)

I'm sure you can think of other ways to store the wind's energy and use it for a speed boost; the interesting thing is that this vehicle can consistently outrun the wind.

Also note that my explanation does not require the vehicle to be on the ground instead of in the water - think "water wheel".

With respect to "tacking": you'll find that Chu-Caroll's article, which smanek linked to, provides the same explanation as I did (skip the initial part where he explains his error and start reading after "let's look at a new diagram"), and mentions the similarity to "tacking" (start at "Once it's moving, what's going on can be described in a bunch of different ways.") However, note that not even the "perfect" sail that Terence Tao assumes in the article linked above achieves more than 2x wind speed; clearly, there is something about this real-world vehicle that makes it move faster than even theoretical (fixed-)sail-powered cars. I think my comment above is a plausible explanation for how this can happen.