What is a hollow cannon (空頭炮)?
These answers restate the rules implemented by our open-source engine, which follows the WXF laws for movement, checkmate and both repetition rulings: a side that forces a repetition by perpetual check or perpetual chase loses rather than drawing. The chase half has not been validated against the official casebook. Where an answer refers to a specific position, that position lives on the linked reference page, where the build verifies it before publishing.
Direct answer
空頭炮 — "hollow-head cannon" — is a cannon on the enemy general's file with nothing at all between them. It gives no check, because a cannon needs a screen. What it does is still unpleasant for the defender: no defending piece may step onto the file between them, because it would become the screen and put its own general in check, and any attacking piece that lands there gives check.
Why it is so unpleasant to face
It freezes a whole file. The defender cannot put a single piece between the cannon and the general, because that piece would be a screen and the move would leave its own general in check, and often cannot remove the cannon. Many attacks are built by establishing 空頭炮 first and then bringing a second piece.
Getting rid of one
The usual answers are to capture the cannon, or to move the general off the file if it can. Blocking is not one of them: a single piece of yours placed between the cannon and your general would be its screen, and a move that puts your own general in check is illegal.
Related questions
Is a hollow cannon giving check?
No. With an empty line there is no capture available, therefore no check.
Is it stronger than a cannon with a screen?
Different. A screened cannon threatens now; a hollow cannon locks the file, because the defender cannot put a piece on it and any attacking piece that arrives there gives check.
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