|Black to move|
1b1r1r1k/1p4qp/p1pBn3/6p1/2P1P3/Q6P/P1B3P1/3RR2K b - - 0 1
a3 protects d6
d1 protects d6
1. ... Bxd6 take with cheapest piece. But that's just an exchange. Misleading variant:
1. ... Bxd6 2. Rxd6 Rxd6 3.Qxd6 Qc3 double attack
Why does 1. ... Rxd6 work?
At first sight, I thought that d8-d1 was the most important line of attack (LoA). But the only important point of it is d6, the point on which you try to mount a superior force.
The most important LoA is b8-h2 though.
The exchange of the rooks diminishes white's influence along the LoA.
1. ... Rxd6 2.Rxd6 Qe5 builds up a battery along the LoA, at the same time outnumbering the defenses of d6 and pinning the rook.
Missing cue: pin while loading a battery