How can a piece that's about to be captured be used as a cannon's screen first?
These answers restate the rules implemented by our open-source engine, which follows the WXF laws and is regression-tested against the official 110-diagram repetition casebook. Where an answer refers to a specific position, that position lives on the linked reference page, where the build verifies it against the engine before publishing.
Direct answer
If a piece is going to be lost anyway, with no safe square to retreat to, moving it onto a friendly cannon's line as a screen can sometimes extract real value from the loss, enabling a capture that piece's arrival makes possible, rather than simply losing it for nothing.
Why this is a genuine desperado idea, not just a consolation
A piece with no future is not really being "sacrificed" in the usual sense, it was already lost the moment it had no safe move. Using its last move to create a cannon capture, rather than retreating to a square that only delays the inevitable, converts a pure loss into a loss that also wins something back. See the desperado piece for the general version of this idea.
What makes this specifically a cannon-screen case
The mechanism is narrower than a general desperado capture: the doomed piece is not doing the capturing itself, it is enabling a friendly cannon to capture through it by landing exactly one square in front of the cannon's target. Recognising that a doomed piece happens to sit on a useful cannon line is the actual skill here.
Related questions
Does this work with an enemy cannon's line instead?
No, moving a doomed piece onto an enemy cannon's line would simply hand the opponent the screen they need; this idea only works with your own cannon.
Is this a common tactic or a rare one?
Reasonably uncommon as a deliberate plan, but worth checking for whenever a piece is about to be lost with no good retreat available.
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