Explanation of Hard Puzzle #34

Question

Wikipedia defines a chess tactic as:

In chess, a tactic refers to a sequence of moves that limits the opponent's options and may result in tangible gain.

I'm having a hard time understanding a tactic from the app Chess Tactics Pro, namely Hard Puzzle #34:

[FEN "6k1/5pp1/P6p/1N6/1P6/2Pn3P/2q1p1P1/1R3R1K w - - 0 1"]

1. Rg1 Nf2+ 2. Kh2 Qe4 3. a7 Kh7 4. Nd4

My questions are:

1. Why is this a "tactic"?
2. Which side is playing the "forced" moves? White, Black, both or none?
3. Why does the tactic end at 4. Nd4?

In essence I would like a plain English explanation of this tactic, so that I can understand it.

Answer 1

This is actually a complicated position (no wonder it's a "hard puzzle"?).

First, we note that White has a substantial material advantage and unstoppable queenside passed pawns. Black has an advanced passed pawn of his own, but it won't be easy to force through. For example if at some point Black plays ...e1=Q, White responds with Rxe1 Nxe1 Rxe1 Qxe1 a8=Q, then White has a decisive material advantage. So in the current position, White is winning, and Black's best hope is to find some kind of perpetual check.

1. Rg1

Forced move. Should be pretty obvious, since White has nowhere else to put the rook.

1...Nf2+ 2. Kh2

Only legal move.

2...Qe4

This move threatens a draw. It's not obvious how it does so, but we'll see more of this later.

3. a7 Kh7

These moves seem innocuous, but they actually matter. 3. a7 interferes with Black's threat because 4. a8=Q+ forces 4...Qxa8, which leaves White with a decisive material advantage. Accordingly, Black gets out of the upcoming promotion with check.

Now we examine what Black's threat actually is. To do this let's assume White passes with no play.

4. -- e1=Q! 5. Rbxe1 (Rgxe1 is similar, while 5. a8=Q?? hoping for 5...Qxa8?? loses the game after 5...Q1xb1 6. Rxb1 Qxa8.) Qf4+ 6. g3 Qd2.

Black threatens 7...Ng4+ 8. Kh1 Qh2#. White can stop the mate, but cannot stop the perpetual. Let's check out an attempt to do so. 7. Kg2. Only move. 7. Rg2?? Qxe1 8. a8=Q Qh1#. 7...Nd3+ 8. Kf3?? (8. Kh1 Nf2+ 9. Kg2 Nd3+ is a draw by perpetual) 8...Nxe1+ 9. Rxe1 Qxe1 and Black wins because 10. a8=Q is met by 10...Qh1+ skewering the queen.

[FEN "8/P4ppk/7p/1N6/1P2q3/2P4P/4pnPK/1R4R1 b - - 1 4"]

1...e1=Q 2. Rbxe1 Qf4+ 3. g3 Qd2 4. Kg2 Nd3+ 5. Kf3 Nxe1+ 6. Rxe1 Qxe1 7. a8=Q Qh1+

Hence the puzzle has White play 4. Nd4. Now if Black goes for 4...e1=Q 5. Rbxe1 Qf4+ 6. g3 Qd2, White has 7. Re2 (Rook is defended by Knight), winning.

Answer 2

Let's sort this mess out:

• This is not necessarily the "clearest" example, especially for beginners.
• Q1: Do the moves limit the opponents options? Well, if running away with the rook from the hungry black pawn doesn't count...
• Q1: Does it result in tangible gain? Well, queening the white pawn gives a gain of +7...
• Thus it is a tactic - maybe you thought "tactics" only refer to mate tactics?
• Q2: "Forced" is a matter of definition. As a mathematician, I must say: Since White is winning, only White 's move are forced. Black can play anything without changing the result. Of course Black will try anything that gives White the chance to err!
• So let's sic Stockfish on em: 1.Rg1 is forced, the next best a7 is drawn. A human might think twice since the white king is in danger, and Rg1 takes a flight. But blatant calculation says: ghosts. Now 1...Nf2+ or 1...Kh7 transpose, let's say the variant is forced for Black, by being superior to alternatives (by 2 pawn units - 1...Qa4 and massacre the N, but the a pawn queens immediately.) 2.Kh2 is forced by the rules of chess, that's a different kind of forcing. 2...Qe4 again is a transposition. 3.a7 (+5.7) now is best, but the immediate Nd4 (+2.8) is (probably) enough for a win, so a7 is not forced. But anyone will play that. 3...Kh7 is not forced at all, in any sense. I'd play Qh4 and pray to all demons of patzing that White overlooks the perpetual. Again, 4.Nd4 is not forced (4.g3 - eh, what?) but 4.Ra1 outright throws the win after 4...e1Q! (5.Raxe1 Qf4+ 6.g3 Qd2, 5.Rgxe1 Qf4+ 6.Kg1 Nxh3)
• Q3: And this is the (rather flimsy) reason why the tactic ends at Nd4. Now in the first Qf4+ variant, White simply interposes at e2, in the second, the perpetual is foiled by f3 being guarded. The danger for the king is over, a7 simply queens, if necessary with some attitude adjustment Ra1. Noone could hinder the puzzle makers to add 4...Qa8 which "forces" 5.Ra1 (of course, Nb5 also wins, if you are low on clock, you can repeat a move after 5...Qe4 :-) Best 5...Nd3 and now almost any non-idiotic move wins and forcing is over.
• Takeaway message: "Forced" can have three meanings: forced by chess law, forced by changing the result otherwise, forced by best resistance.

Answer 3

Both sides are forced to a degree. White's rook on f1 has to stay on the back rank to protect the other rook, and g1 is the only safe space.

Both players have pawns that are close to promoting, so Black needs to keep the White pawn from promoting while protecting their own pawn. But as they have no hope of promoting their own pawn, they need to keep up threats so White can't work on promoting their pawn. ...Nf2 does that, and results in Kh2 being forced. From there, there's not much Black can do. Black's knight is too far away to stop the promotion, with it being a queen versus two rooks, guarding the promotion square just means trading the queen for a pawn and a rook, which is losing.

... Kh7 at least keeps the promotion from being with a check, but Nd4 threaten to capture the pawn on e2. If Black responds by capturing the knight, that leaves a8 open for the pawn to promote. And if Black doesn't respond, then White has three passed pawns, while Black has no pawn threats of their own. As long as Black's pawn is on e2, White's options are limited as they need to keep their forces ready to stop a promotion. As soon as that threat is gone, White's forces are free to force a promotion of their own.

Answer 4

Chess tactics and chess puzzles aren't synonyms.

A typical online chess puzzle may only have one unique sequence of moves that keeps you ahead. So for this we only need to explain each of the moves separately.

In a puzzle you make a solution against the moves that the opponent plays. So you don't think if the move black plays here are forced. You can think of the moves that black could play.

⁣1. Rg1
Rook is attacked, and there is no other option that keeps it and prevents promotion. There also is no clear benefit for saccing it.

⁣1.. Nf2+ 2. Kh2
There is no option.

⁣2.. Qe4. 3. a7
Now we need to look at alternatives, like why doesn't 3. g3 and then 4. a7 also win?
Well after 3. g3 Qf4+ 4. g3 Qf5 our knight is hanging. 3. a7 keeps black busy because he can't play Qf5 anymore allowing us to check his king.

⁣3.. Kh7. 4. Nd4
Now black has prevented our checks and promotion. If we want to force a promotion, we need to play Rh1, but can we? After 4. Rh1 Qf4+ 5. g3 Qf5 6. Kh2 Ne4 white is in trouble, this can't be avoided so white just needs a defensive move and that was 4. Nd4.