The idea is to make sure black's dark squared bishop can't move (also avoiding any promotion threats), and threatening either Qg3 (with the white bishop on f1, zugzwanging white) and Kg3 with mate on the h-file, maneuvering such that a dark square on the h-file can be reached by the queen.
1.Qb8 Bb5 (Bd3 then 2.Qf3! Bf1 3.Qg3 wins) 2.Qg4 Bc6 3.Qd1 Be4 4.Kg3! Bg6 5.Qc1 (or Qa1) Qh5 6.Qh1!(foring the bishop to move away from the h-file) followed by Qh8+ and win
or another try
1.Qb8 Bc4 2.Qe5 Ba6 (Bd3 then Qg5) 3.Qc7 Bd3 (Be2? 4.Qg7 Bf3 5.Qa1 followed by Kg3 and as above) 4.Qf4 Be2 5.Qg5! Bf3 6.Qc1 Be2 7.Kg3 and same as above
The variations are numerous, but the ideas are the same, and white cannot avoid the maneuver.