A nasty tactical trick which often wins on the spot. White has a powerful f5-knight which is lined up along the h3-c8 diagonal with an undefended Black queen on d7. The White f5-knight also controls the critical g7-square, which is defended only by the Black g8-king.

By playing 1 ♕g4! in these positions, White creates a winning double threat. The immediate threat is 2 ♕xg7#. But there is also a secondary threat, that of winning the undefended Black d7-queen with the discovered attack with check 2 ♘h6+ and 3 ♕x♕.

Hence the move 1 ♕g4! exploits the two weaknesses inherent in these positions. Firstly the White queen coordinates with the White f5-knight to threaten mate on the g7-square. Secondly the White queen places herself on the same h3-c8 diagonal as the undefended Black d7-queen to threaten to win the Black queen with a discovered attack with check.

Black then loses either his king or his queen.