From what I've read, the issue is as simple as performing step 3 of a 4-step handshake and the consequences of performing that step more than once. Considering the complexity of these kinds of algorithms, I'm somewhat surprised that it is so 'simple' of a concept.
How can it be that a system of this complexity was designed without anyone thinking about what would happen if you performed the step twice? In some sense, it feels like this should have been obvious. It's not really a subtle trick, it's a relatively blatantly obvious defect, or at least that's the impression I'm getting.