The black rook guards against Bg3 and Rxh4 mate. A move of the c4 knight will threaten the deflecting 2.Bf1+, but Black can prepare a stalemate defence, which White must forestall by placing his knight en prise. Hence the main line 1.Sa3 Bh1 2.Bf1+ Rg2 3.Bg3 bxa3 4.Rxh4. The idea was probably not new in 1930, but needing four black pieces to be immobilised for the stalemate trap to work may have added a touch of originality.