Wednesday, February 18, 2015

Calculating Forcing Moves: Defensive Idea 2, Part 1 (Block)

When your opponent creates a (real) threat, or you are considering creating one against your opponent, there are five defensive ideas to consider.  We have already looked at three different ways to implement the first defensive idea of going after the attacking piece:  capture the attacking piece, pin the attacking piece, or deflect the attacking piece.  This post will explore the second basic defensive idea of blocking the attack.

The idea here is pretty straight forward -- you put something less valuable on one of the squares between the attacking piece and the piece being attacked -- but there are some interesting subtleties to keep in mind.  When there is an immediate threat on the board, it is often very easy to find specific moves that satisfy this idea.  The search method is simply to look at the squares between the two pieces, and see what pieces/pawns you can place on any of those squares.  Those moves go onto your candidate list to calculate further.

Blocking with similar pieces is more forcing than blocking with pieces that don't attack back, and sometimes you can gain a tempo by blocking this way.  We'll look at an excellent example of this below.

Also, when the threat is not immediately on the board but is on the next move (like a mate-in-1 threat), you can "prepare" to block by rearranging your pieces.  We'll look at an example of that in another post.

The general process is to evaluate the position, identify all threats, and then to use the five defensive ideas to identify specific candidate moves.  Next, calculate each candidate move to the end and evaluate the final positions (material and activity).  As you progress through the candidates, keep in mind the "king of the hill" (KOTH) candidate move to help you pick the best at the end.

Let's start with a simple position to demonstrate the basic concept in the second defensive idea.  White just played Bd2 attacking Black's knight.  Black to move.

3N4/4p3/8/npp2k2/3b4/3P2P1/P2B4/7K b - - 0 1

Let's evaluate the position and then find all of white's threats.  Material is equal, bishops are on the same color, but black's pawn structure is better and his king is more active than white's.  Black has an advantage and should be playing for a win, while white is currently playing for a draw.  White's immediate threat is to capture black's knight with Bxa5.  Let's look at the five defensive ideas to see what candidate moves we find, and then pick the best move.

Idea 1 (capture, pin, deflect):  none.
Idea 2 (block):  searching square by square between the bishop and the knight, we see black can put his bishop on c3, or his pawn on b4.  1...Bc3 counterattacks white's bishop, but it is unsafe and appears to simply lose the bishop, so we'll only consider 1...b4
Idea 3 (move):  the knight has four moves, none of which appear safe.
Idea 4 (defend):  black cannot add a defender to the knight (aside from Bc3, which we already considered).
Idea 5 (counterattack with an equal or greater threat to WIN, DRAW, DEFEND):  an equal threat would be to attack white's bishop or knight, which isn't an option here.  A greater threat would be to attack a rook, queen, give check, or threaten promotion.  1...c4 looks like it might threaten promotion.

The easiest move to calculate is 1...b4, after which neither side has any immediate threats.  Black might try to trap white's knight, or try to promote a pawn, but the immediate threat has been addressed and black has an edge due to his pawn structure and more active king.  This is our KOTH.

Let's see if 1...c4 leads can force anything better for black than our KOTH.  After 1...c4 2. Bxa5 (most forcing reply) black now has 2...cxd3 and 2...c3 but after both moves white can force a blockade and remain up a piece with no chance of black promoting, so then 1...c4 2. Bxa5:

If 2...cxd3 3. Bd2 and white is winning.  Not better than our KOTH.

If 2...c3 3. Nc6 and now the bishop must retreat while also defending the c-pawn, so 3...Bf6 4. Nb4 followed by 5. Nc2, and white is winning.  Not better than our KOTH.

So 1...b4 blocks the attack, and allows black to keep his edge.

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