Lesson
White to move. Main line: 1. Nf3+ Kd5 2. Bb3+ Ke4 3. Ke2 Kf5 4. Ke3 Kf6
A bishop only ever covers one colour, so mate can only be delivered in a corner of that colour. A defender who understands this runs for the wrong corner, and the whole technique is about not letting that happen.
The knight is the slow piece and it does the awkward work: it covers the squares the bishop cannot reach, which means it has to be repositioned constantly as the king is driven along the edge.
The tablebase says mate in 57 from this position. The fifty-move limit counts single moves for each side, so there is room — but far less than in any other basic mate, which is why this one is worth drilling rather than reasoning out at the board.
The one basic mate with a deadline
Every other elementary mate is comfortable. This one is a race: the tablebase gives mate in 57 from the position shown, and the fifty-move rule counts moves for each side, so the margin is real but thin. A player who has not drilled it usually does not finish in time.
That alone makes it worth learning properly rather than reasoning about at the board — it is the classic example of an ending where knowledge and strength come apart.
Why the corner matters
A bishop covers one colour, permanently. Mate can therefore only be delivered in a corner of that colour, and a defender who understands this heads straight for the wrong one.
So the technique has two phases that are usually taught as one: drive the king to the edge, then walk it along the edge to the correct corner. The second phase is the hard one, and it is where the knight does its awkward, repetitive repositioning.
The knight is the slow piece
A knight cannot lose a tempo and cannot cover a square adjacent to the one it stands on. Every time the barrier needs to move, the knight has to be brought round, and the king must hold everything together while it travels.
This is why the ending feels fiddly compared with two bishops. Nothing about it is deep; it is precise, which is a different kind of difficulty.
1.Nf3+ (the line shown): The tablebase’s own move — mate in 57 with best play from both sides.
Aiming for the wrong corner: No mate exists there. The bishop cannot control the mating square, so the defender is safe.
Mate is only possible in a corner the bishop’s colour controls — the defender will run for the other one.
Two phases: to the edge first, then along the edge to the right corner.
Count the moves. Fifty-seven against a fifty-move limit means this must be known, not worked out.
Source: Syzygy endgame tablebases (via the Lichess tablebase API) — https://tablebase.lichess.ovh/standard
Source: The fifty-move rule — https://en.wikipedia.org/wiki/Fifty-move_rule