Make 5 stones at one line. Build five black stones at one line. You need to build 5 stones on vertical, horizontal or diagonal line. Do not build five white stones. Unlike the case of five stones on the second line in the corner, ... Even if the extra black stone is one line away, Black can still kill using the small. The best black can do is to make a small reduction in sente. ( can also be played at a, but that gives white an extra ko threat.)