There's a small blue light on the Big Board that is activated by an Assistant Producer when Alex is done with the question. It's not visible to the TV audience. The key is knowing the timing between the reading of the question and the activation of the blue light. Returning players have a distinct advantage in this regard through experience. One of the strategies that James employs is to take as many of the high-value questions off the board before the "new" contestants get the hang of the timing. By then, he has a huge lead.
As I understand it - Alex does not read the answers.
Every answer on every category has been pre-read by Alex and when a player selects an answer, the Big Board as you call it plays back the answer.
I have not been on the show but I believe (I think I read it somewhere) that recently they have been giving new contestants more practice time than before to let them familiar with the buzzer timing.
It is sad to watch the antics of people trying to ring the buzzer with the wrong timing...I saw one lady who kept frantically pointing her buzzer toward the board, as if that would help her.
I happened to start watching just as Holzhauer started his streak. He intentionally causes cognitive dissonance by wandering around on the board, but there’s no law against it and he’s got a knowledge base bigger than mine for sure. I don’t particularly like him, but I respect his success.