What a beautiful motive, first I calculated Rb8, which traps queen after Qc6, but not after Qa5. Thought there is nothing for short while, but I dug deeper into original variation and its there... really beautiful puzzle.
- ... a5 - threatens c6 and trapping queen, which is inevitable, but white have two reasonable defensive options:
attempt to escape - 2. Qc6 Ra6 3.Qd5 Rd6 and queen is trapped on d5 - i don't think I ever seen this before.
or try counterplay - 2. Bg4 c6 3. Bxd7+ Kxd7! 4. Qb6 Ra6 and queen ends up trapped in this variation too.