What is a double cannon mate (重炮) and how does it work?
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
重炮 (the double cannon mate) stacks two friendly cannons on the enemy general's file, one directly behind the other, using the front cannon as the rear one's screen. So the rear cannon gives the check. It is mate when the front cannon is defended, so the general cannot take it, nothing can be interposed, and the general's own advisors block its way sideways.
Why it is so hard to defend once set up
A single cannon aimed at the general is normally something a defender can block with any piece. With two cannons stacked, a piece put between the front cannon and the general blocks nothing: it becomes the front cannon's screen, so the front cannon would check instead, and the move is illegal. In the usual mating position the front cannon stands directly in front of the general, leaving no point to interpose on at all. See the double cannon mate for the position-by-position mechanics.
Getting the setup in the first place
The harder part is usually not the mate itself but manoeuvring two cannons onto the general's file against a defender who can see it coming, which typically requires other threats to distract the defence first. It is rarely the opening move of a combination; it is usually the point everything else was building toward.
Related questions
Can the defender ever safely block a double cannon setup?
Not between the front cannon and the general, since a piece there becomes the front cannon's screen. The real defences are capturing the rear cannon, the general taking the front cannon when nothing defends it, or the general stepping off the file.
Does this require the file to already be fully open?
The rear cannon needs exactly one piece, the front cannon, between itself and the general. Any other piece on that stretch of the file means there is no check.
.