There is a short interview to establish personality fit. Then the actual hurdle follows, an interview with the founder with the goal to see how you react under pressure. There are a couple of "thinker" questions that, considering the situation, are rather difficult to answer and there is a process to them. Of course you do not get time to slowly go through it, but you are tested on a) how well you think under pressure, b) your demeanor in light of the (often annoying) challenge and c) logical thinking