user4277: I am trying to understand this position a bit better and would appreciate some help in doing so. I am black and it's my turn... 
I played the move ..c5. I don't have a deep understanding of this specific position, hence my question (I'm ELO 1700) but my thoughts were along the lines of preventing white to get Bb4 in and at the same time gaining a tempo by kicking the knight. This then also allows me to bring my knight back into the game without obstructing any of my rooks. 
For some reason this move was extremely bad (listed as the 10th best move with a relative score of 2.0 points lower than the best move)
Can someone please explain to me why this move is so bad? 
[title "Black to play"]
[fen "1n1qr1k1/pr3ppp/2p2b2/8/2PN1Q2/2BP2P1/PP2PP1P/3R1RK1 b - - 0 20"]

1... c5


user15509: I like c5 for these positional reasons: 

You are one step closer to activating your knight, which you cannot at the moment because the pawn would hang.
Forcing a bishop trade that will allow you to put the rook on the 7th and attack both e and a pawns, also avoiding the N fork on d6.
Reducing the number of attackers on your K (minus the bishop), while not reducing your defences because Q can perform bishop's duties from f6.

20...g5, barring computer tactics is suicidal IMO, because you are undeveloped with a kingside attack on the way, and what do you do, expose your King some more?

user3757: To add some information to @overtheboard's answer, after c5 you are inducing Nf5, and you end up with three white pieces (Bishop, Knight and Queen) attacking you kingside. Also, since then white is menacing Nd6, forking both your rooks, you're not gaining a tempo, but giving a tempo to white and you do not have the time to activate your own knight.

user7500: The reasons for not playing 20...c5 are mainly tactical. 20...g5 21. Qg4 h5 22. Qxh5 Bxd4 23. Bxd4 Qxd4 24. Qxg5+ Qg7 wins a piece for two pawns. Since you are already down three pawns for nothing, it won't do to just sit around and wait.
[fen "1n1qr1k1/pr3ppp/2p2b2/8/2PN1Q2/2BP2P1/PP2PP1P/3R1RK1 b - - 0 20"]

1... g5 2. Qg4 h5 3. Qxh5 Bxd4 4. Bxd4 Qxd4 5. Qxg5+ Qg7


4277: <p>I am trying to understand this position a bit better and would appreciate some help in doing so. I am black and it's my turn... </p>

<p>I played the move <code>..c5</code>. I don't have a deep understanding of this specific position, hence my question (I'm ELO 1700) but my thoughts were along the lines of preventing white to get <code>Bb4</code> in and at the same time gaining a tempo by kicking the knight. This then also allows me to bring my knight back into the game without obstructing any of my rooks. </p>

<p>For some reason this move was extremely bad (listed as the 10th best move with a relative score of 2.0 points lower than the best move)</p>

<p>Can someone please explain to me why this move is so bad? </p>

<pre><code>[title "Black to play"]
[fen "1n1qr1k1/pr3ppp/2p2b2/8/2PN1Q2/2BP2P1/PP2PP1P/3R1RK1 b - - 0 20"]

1... c5
</code></pre>
 15509: <p>I like c5 for these positional reasons: </p>

<ol>
<li>You are one step closer to activating your knight, which you cannot at the moment because the pawn would hang.</li>
<li>Forcing a bishop trade that will allow you to put the rook on the 7th and attack both e and a pawns, also avoiding the N fork on d6.</li>
<li>Reducing the number of attackers on your K (minus the bishop), while not reducing your defences because Q can perform bishop's duties from f6.</li>
</ol>

<p>20...g5, barring computer tactics is suicidal IMO, because you are undeveloped with a kingside attack on the way, and what do you do, expose your King some more?</p>
 3757: <p>To add some information to @overtheboard's answer, after c5 you are inducing Nf5, and you end up with three white pieces (Bishop, Knight and Queen) attacking you kingside. Also, since then white is menacing Nd6, forking both your rooks, you're not gaining a tempo, but giving a tempo to white and you do not have the time to activate your own knight.</p>
 7500: <p>The reasons for not playing <code>20...c5</code> are mainly tactical. <code>20...g5 21. Qg4 h5 22. Qxh5 Bxd4 23. Bxd4 Qxd4 24. Qxg5+ Qg7</code> wins a piece for two pawns. Since you are already down three pawns for nothing, it won't do to just sit around and wait.</p>

<pre><code>[fen "1n1qr1k1/pr3ppp/2p2b2/8/2PN1Q2/2BP2P1/PP2PP1P/3R1RK1 b - - 0 20"]

1... g5 2. Qg4 h5 3. Qxh5 Bxd4 4. Bxd4 Qxd4 5. Qxg5+ Qg7
</code></pre>
