Purpose behind the move I was just guessing the best move  in the given position(black to move) .
I thought it would be 'Be6' , But the engine gave 'c5' , which weakens the 'd5' square .
I want to know the plan behind the move 'c5' if it is the best move.
The move c5 is obviously anti-positional. So the purpose must be tactical.

[Title "Anti-positional c5"]
[FEN "1rb2rk1/5pb1/p1pp1np1/q6p/2PBP2P/2N2P2/PP1Q2P1/3RKB1R b - - 0 1"]

1... c5 2. Be3 Rxb2 3. Qxb2 Nxe4 4. Bd2 Nxd2 (4... Bxc3 5. fxe4) (4... Nxc3) 5. Kxd2


1…c5 2.Be3 Rxb2 3.Qxb2 Nxe4 is the obvious try. But after 4.Bd2 the position is pretty tricky because 4…Nd2 fails to 5.Kxd2, Bxc3 fails to fxe4 and Nxc3 steps into a selfpin. But the selfpin cannot be exploited by white because black has Re8+/Qe5+ coming up. This looks like a pretty crushing attack.

If white prefers to exchange the Bd4 for the Nf6, then the weak d5 square is more than balanced by the weak black squares in whites position. Bd4 is a much bigger threat than Nd5.