The problem
The kings face each other on e5 and e7. If it were Black's move, the black king would have to step back to d8, e8 or f8, and the white king would walk in to d6 or f6. Every black move here loses.
But it is White to move, and White has no pawn move to spare: c5 is blocked by c6 and e6 by the black king. A position in which the side to move is worse off is called a zugzwang, and White needs a way to hand the move to Black.
The triangle
White's king walks round three squares that touch each other: f5, e4 and f4. In the tablebase line the black king answers by stepping from e8 to d8 and back.
Three moves for White against two for Black means that when the white king returns to f5, the same position has arisen with Black to move. After Ke7 Ke5 the starting position is back on the board, now with Black to move, and the white king breaks in.
Kf5 at the start and Ke5 on the way back are the only winning moves. All three other first moves draw.
Typical mistakes
Shuffling the king between two squares achieves nothing: after two moves each, the same position returns with you to move again. Only a route of three squares, against a defender with two, gains the tempo.
Do not drift too far away. In the demonstration, retreating to e3 or f3 instead of completing the triangle lets the win slip.
Remember
- Recognise the zugzwang
- Three squares against two
- Return with the opponent to move
- Use it when no pawn move can spare a tempo