Wasn't easy for me to spot this idea, but I did eventually stumble onto the right plan, but by accident. Not sure I am not missing something much better, but 1...Rb1 looks very good to me- it allows black to take complete control of the 2nd rank:
White can't take the rook- after black retakes with the bishop, white has a rook and a bishop en prise and must lose a piece. This means the rook a2 has to retreat to defend the pieces on the first rank:
And white can't prevent the rook one e8 from reaching e2. With doubled rooks on the second rank, black should win.