Does it just copy Red?

Mirroring keeps things equal, but Black can later deviate (e.g., with a counter-cannon) to seize the initiative.

Mirror Soldier (Dui Bing Ju) vs Adjacent Soldier

The symmetric reply to Adjacent Soldier: both sides push a soldier, keeping the position balanced and the centre closed.

Red's 兵三进一 starts the line.

Mirror Soldier avoids early theory and leads to a quiet, maneuvering game. It is a safe choice for players who dislike forced lines.