If the problem is too hard it's not always possible to talk or draw something while thinking about it. You go through dozens of possible solutions in your head, to find a good one.

That's like asking a champion chessplayer to explain what he's thinking about while trying to make a move.

So this only works if the candidate already knows the solution, and knows which way is right and just explains that one (and maybe a few bad alternatives on purpose). Or if the problem is relatively easy.