Black pushes his King's Rook's Pawn
out to KR3(h6) to attack and dislodge White's bishop, and
safeguards against any later B-Q3 and Q-B2.
B-Alt. White's dark Bishop under attack withdraws to his
KR4(h4), maintaining the Kt+Q pin.
White Black
1. P-Q4 Kt-KB3
2. P-QB4 P-K3
3. B-Kt5 P-KR3
4. B-R4