I was surprised by the sacrifice is the major for the captain to discover the general early. But then that's probably why I'm merely meager general material right now.
Here was my thinking into this: I was already down a captain, so by taking the captain I get even in captains but am down a major instead. All else equal, my opponent would have a one high piece advantage in the end, but at that point it won't matter whether it is a major or a captain. What I gained in the exchange is the information of the general's location. I didn't take a material advantage of this knowledge very well, but it did make the marshal's location more obvious, as it was the only logical piece to be protecting a major on the other side, so the info led to the discovery of even better info. Now, I wouldn't have been happy if I had instead found only a colonel, but since I already knew one colonel I figured I was more likely to find a general or marshal.