1.Qe4, with two threats that are never realised 2.Sxb3, Bxb3.
1...Ba4 2.Rxa4
1...Ba2 2.Bxa2
1...Bxc2 2.Sxc2
1...Bxc4 2.dxc4 (Set 2.Qc5)
The queen must stop guarding d8 to prepare for Sf6 mate after the bishop is neutralised.
The point of the problem lies in the alternative queen moves which fail because White has set up a battery which, if opened, will result in the wrong side being mated!
1.Qa5? Ba4! (2.Rxa4?)
1.Qc5? Ba2! (2.Bxa2?)
1.Qe5? Bxc2! (2.Sxc2?)
1.Qf3? Bxc4! (2.dxc4?)
Richard Stein: The principle turned out to be simple — don’t move the queen where it establishes a battery.