A FOUR threatens five next move — the opponent must respond, like a check in chess, and nothing else on the board may be attended to until it is answered. An OPEN FOUR threatens five in TWO ways at once: two five points, one defensive move — effectively mate. Here black's trio at (5,7)–(7,7) meets white's cap at (10,7): playing (8,7) makes the rush four with its single five point at (9,7), and white must spend the stone. Rush fours are how a player buys tempo; open fours are how a player wins.
Finish this step to move on.