All you need to know is when the crucial moment in the game appears then you can think of several lines of games from that point and you'll figure out the best move.

But spending that much effort on every move is mind numbing and exhaust you in no time

So a cheater only needs a signal to tell whether the current position is the decisive one in the game and then think very hard on this one before playing the best move.

So someone can run an engine and feed that as some signal to the player and then you win and win.