After 1...g3 the king will be free to move between h2 and h3. To what use can White then put his black-squared bishop?
1.Qa7! g3 2.Bb6 Now with the support of the queen the bishop can control h2. 2...Kh2 3.Bg1+ Kh3 4.Qd7. If 2...Kg4 3.Qd7+ any 4.Qf5. A famous example of a standard problem idea known as Turton-doubling.
Challenge Problems 