1.Bb7? threat 2.Be4 fails to 1..e1Q. 1.Ba6 threatens 2.Bd3 and if Black
replies with 1...e1S 2.Bb7 now works, however Black has 1...Sb5! and after
2.Bxb5 e1S 3.Bc6? fails because after 3..Ke6 4.Be4 the king can escape
to f7. White must close this bolt hole, so 1.g3! threatens 2.Sxg7+ Kg5
3.Sh3+ Kh6 4.Sf5, and Black’s only defence is 1...Rf7, guarding f5. Now
everything works. 2.Ba6 (threat 3.Bd3) Sb5 3.Bxb5 (threat 4.Bd3) e1S
4.Bc6 (threat 5.Be4) Ke6 5.Be4; 2...Sc4 3.Bxc4 (threat 4.Bd3) 3...e1=S 4.Bd5 & 5.Be4
Forcing a promotion to knight or queen in order to eliminate the
possibility of the other promotion is called the Holst theme. 1...Sb5,
which decoys the bishop so that it moves to a different square on the
same line, creating a weakness, demonstrates White Roman strategy.