You can buzz in as soon as Trebek stops speaking. If you buzz early, you can’t buzz again for 0.2 seconds. Watson gets an electronic signal when it is time to buzz, and then physically presses the button. The humans get a light, but they don’t bother looking at it, they try timing when Trebek will finish. I think this is a serious advantage for Watson.
I agree that this is a big advantage to Watson. However, I don't understand why it works this way. The reason you can't buzz early is because it makes the game annoying to watch and unpleasant to participate in. Alex would constantly be interrupted as the contestants race to buzz in and cut him off.
However, making contestants hit the buzzer just when Alex finishes adds a substantial dexterity element to the game. Watson has an advantage in this dexterity game, but who cares? Everyone wants to know if Watson is smart, not if he can press a button with millisecond precision.
A better way to handle the buzzer, it seems to me, would be to let people buzz in early but not to count it until Alex finishes the question. At that point, if more than one person has buzzed in, the winner is selected randomly. Otherwise, it's a speed race, with no precision to the timing. This slight change in the algorithm should remove the precision timing from buzzing in and make it more of a race to figure out the answer the fastest.