Tuesday, May 26, 2015

Calculating Forcing Moves: Defensive Idea 2, Part 2 (Prepare to 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 piecepin the attacking piece, or deflect the attacking piece.  In an earlier post we looked at the second defensive idea of blocking the attack.

In this post we will look at a position where the threat is not immediately on the board but is likely to be played on the next move (like a mate-in-1 threat), and we will use the idea of "prepare" to block" by rearranging our pieces to allow a block if the threat is executed.  

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.

The next position uses the same basic defensive idea of blocking an attack, but also demonstrates how blocking with a like piece can gain time and be more forcing than blocking with other pieces.

Black just played Qe6+.  White to move.

r4r2/4ppBk/p2pq1p1/1p5n/4P2P/P4P2/KPPQ4/3R1BR1 w - - 0 1

Let's evaluate the position and then find all of the threats.  Pawns are equal, both sides have one queen and two rooks, and although white is currently up a minor piece his bishop on g7 is en prise and he is in check.  Will white have time to save his bishop?  Black's threats are the check, and then to capture the g7 bishop on his next move, after which material will be equal.  White is also threatening checkmate with Qh6#, but needs to address both the check and the threat to his g7 bishop.  Let's look at white's three defensive ideas (you can't defend or counterattack when in check!) 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 queen and the king we find four moves that block the check: 1. Qd51. c41. Bc4, and 1. b3.
Idea 3 (move):  1. Ka1, and 1. Kb1
Idea 4 (defend):  n/a
Idea 5 (counterattack with an equal or greater threat to WIN, DRAW, DEFEND):  n/a

Let's look at the easiest to calculate moves first (instead of the most forcing).  After both king moves, black just captures the bishop (if 1. Ka1/Kb1 Kxg7) and the position is materially equal.  This is our KOTH.

Now let's look at calculate each block and see if we get anything better than our KOTH evalulation of equal.

If 1. Qd5 white has no threats (because Qh6# is no longer an option) and black can simply take the bishop, or can take the queen first with check and then the bishop.  Black has nothing better, and this line is no better (and no worse) than our KOTH.

If 1. c4 white now has one threat to deliver mate with Qh6#.  Black can easily address the one threat threat by capturing one of the attackers with Kxg7.  Black has nothing better, and this line is no better (and no worse) than our KOTH.

If 1. b3 again white now has one threat to deliver mate with Qh6#.  Black can easily address the one threat threat by capturing one of the attackers with Kxg7.  Black has nothing better, and this line is no better(and no worse) than our KOTH.

If 1. Bc4 white now has two threats - to capture the queen, and to deliver checkmate.  The checkmate is the greater threat so lets look at black's defensive options.

Idea 1:  Black can capture the g7 bishop with 1...Kxg7 or 1...Nxg7.
Idea 2:  Black can block the white queen from getting to h6 with 1...Nf4 and 1...g5.
Idea 3:  Black can defend the mating square on h6 again with 1...g5.
Idea 4:  The black can escape with 1...Kg8.
Idea 5:  Black's counterattack ideas must begin with a check, given white's checkmate threat, and black only has one:  1...Qxc4+

Of black's six candidate moves we can quickly dismiss five -- the two captures, the two blocks, and the move -- because they all immediately lose the queen.  That only leaves black's one counterattack idea, so after 1. Bc4 Qxc4+:

We have the same exact same situation as in the starting position, except now white is down an entire piece and, as soon as white addresses the check, black is going to capture the g7 bishop.  This would leave white down an entire piece and appears to refute white's 1. Bc4, no?

Here again white has two king moves (both of which leave white down a piece, and is worse than our KOTH), and one block with his tiny pawn.  However, that pawn block comes with a punch, because it is again attacking black's queen!  So after 1. Bc4 Qxc4+ 2. b3:

After this simple block by the simplest of pieces, the roles have again been reversed and now black has to deal with two threats!  The checkmate threat takes priority, and no matter what black tries the best he can do is go desperado and snatch a pawn and the g7 bishop for his queen.

This position demonstrates very clearly the power of blocking with a like-piece, and how you can turn a simple pawn block into a game-winning counterattack!  This position also shows the risks in counterattacking!  Looking at the original position, black should have calculated his move Qe6+ a bit longer.  It clearly would have been much safer for black to have just captured the g7 bishop instead of going for a some fancy counterattack/intermezzo!!

Finally, the last position demonstrates additional blocking options when the threat is one move away.  In this position you can expand the block concept to moves that "prepare to block", by either moving a piece into position to block once/if the threat is played, or by moving the king out of the way to allow a piece to block.

I'll leave this last one as a puzzle.  It is white to move:

5rk1/2p4p/Bpn1p3/p2PP1q1/3PP3/PP3p2/1BQ2P1r/2RR1K2 w - - 0 1

[HIGHLIGHT TO SEE ANSWER:  White's only defense is to move his king to allow the a6 bishop to block: 1. Ke1 Rh1+ 2. Bf1]

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