I'd suspect they have to be using a cache, too, but I tried a whole slew of random torrents off kickass.to, and if it played it at all, I could scrub every single one of them to any point in the video with a mere 1-2 sec delay. So, they've either cached a gi-normous amount of torrents, or they've come up with something really interesting in the ability to dynamically stream any block with very low latency.