The Move With No Explanation
A rook steps back one square and the evaluation jumps. Four categories that cover almost every incomprehensible engine move, and how to work out which you have.
· 3 min read
Every so often an engine produces a move that looks like a mistake. A rook drops back to its starting square. A pawn advances one square in a position with no obvious use for it. The king takes a step sideways with no check in sight, and the evaluation improves by a third of a pawn.
These are not errors, and they are not incomprehensible. They fall into four categories, and identifying which one you have takes a couple of minutes.
Prophylaxis
The move stops something you had not noticed was a threat. Not a threat now, a threat in four moves, once the opponent has finished the regrouping they are halfway through.
This is the largest category and the hardest to see, because the thing being prevented never happens, so there is no evidence of it in the game. To find it, play the position without the engine's move: make a null move for your side and ask what the opponent would do with a free tempo. The answer is usually what the strange move was about.
Waiting
In positions where every move slightly weakens something, the side who must move first is worse. So the engine plays the move that changes least, handing the obligation back.
The signature is a move that appears to do nothing at all, in a position that is not otherwise blocked. Rook shuffles along a rank, a king step between two equally good squares. If the evaluation improves when nothing has changed, the improvement came from whose turn it is.
Preparation with a long fuse
The move is the first step in a plan that takes eight moves to become visible. A pawn to h3 that creates a square for a knight, that will use the square to reach an outpost, that will make a break possible on the other wing.
| What you see | What it probably is | How to check |
|---|---|---|
| A move that stops nothing and does nothing | Waiting | Whose turn would you rather it be? |
| A defensive-looking move with no threat present | Prophylaxis | What would the opponent do with a free move? |
| A small pawn move on a quiet wing | Preparation | Play twelve moves of the line |
| An ugly move that improves a bad piece | Repair | Which of your pieces was worst placed? |
Repair
The least glamorous and most common. Your worst-placed piece is dragging the whole position down, and the engine's move improves it even though the move looks backwards.
Human players systematically underrate this. We look for moves that do something to the opponent, and an engine has no such bias. If you cannot see why a retreat is good, check whether the retreating piece had any future where it was.
The one that is not any of these
Occasionally an engine move is a horizon artefact rather than a plan. The tell is that it stops being the recommendation as depth increases.
That is worth checking before spending twenty minutes trying to understand a move. Deepen the analysis, and if the move survives, it is real and worth the effort. If it evaporates, you were trying to understand an unfinished calculation.
Why this matters more than the evaluation
Understanding one strange move properly is worth more than reading a hundred evaluations. Prophylaxis and waiting moves are the two areas where strong human players differ most from weak ones, and they are exactly what an engine will show you if you ask the right question of the position rather than reading the number and moving on.
Common questions
- Why do chess engines play moves that make no sense?
- Usually because the justification is beyond the depth a human calculates. The move prevents something, prepares something, or waits for the opponent to commit. Playing the engine's line out on a board reveals the reason in most cases within ten moves.
- Should I play moves I do not understand?
- Not in a game. A move whose purpose you cannot state will be followed by moves you cannot find, and the position will go wrong two or three moves later. Understand it in analysis; play what you understand at the board.
Try it on your own position
The analysis board runs the same engine in your browser, free. The app adds what a web page cannot: reading a position straight from a photograph, saved boards, and analysis with no connection.
