This problem was composed in 1917. The first appearance of a long-range clearance move (a white piece making way for a like-moving white piece to barrel down the same line was shown by Frank Healy in a three-mover setting in 1861 in a composition tourney in Bristol. Hence the theme came to be known as Bristol.)
The key is somewhat surprising if you have not seen this type of paradoxical idea before. The bishop retreats all the way to the other corner anticipating someone else coming from h8 to b2. 1.Ba1. Black has no choice. 1...Kxg2. Now white promotes to queen. 2. h8=Q. Again, Black has no choice 2...Kf2. Now, we see the point of the clearance. 3. Qb2#. Note how the King at e4, the black pawn at g3, all have their roles in restricting the black monarch's freedom. There is one forced line of play throughout.
The Bristol theme here was characterized by Bh8->Ba1, and then Qh8->Qb2
For more examples with Bristol moves used multiple-times in the same problem, in post-key phase, in try play, or in helpmates, see the excellent ozproblems page on Bristol