Promotion: for almost every piece, three ranks deep

Try it on the board

The zone is three ranks, and touching it is enough

The three ranks furthest from you - a, b and c - are the promotion zone. A piece may promote at the end of any move that ends inside the zone, starts inside it, or crosses into it. Leaving the zone counts too, which surprises everyone: a rook that steps back out of enemy territory may still flip on the way.

Physically it is one action: you turn the tile over, and its other face carries the promoted kanji in red. On this site the promoted piece is drawn in red for the same reason.

You never choose what you get

Each piece has exactly one promoted form. Rook becomes the Dragon King (龍), keeping its rook move and gaining the king's step on the diagonals. Bishop becomes the Dragon Horse (馬), keeping the diagonals and gaining the king's step orthogonally. Both are enormous upgrades and both are still not queens.

Silver, knight, lance and pawn all become gold generals. A promoted pawn is called a tokin (と) and is one of the best attacking pieces in the game: it costs almost nothing, it moves like a gold, and if your opponent captures it, it goes back to their hand as a mere pawn. King and gold never promote.

Optional, except when it is not

Promotion is your choice, and declining is a real decision rather than a formality. An unpromoted silver keeps its backward diagonals, which a gold does not have, so silvers are often left alone. Knights and lances are frequently left unpromoted so they keep threatening squares a gold cannot reach.

It becomes compulsory only when the piece would otherwise be stuck forever: a pawn or lance reaching the last rank, a knight reaching either of the last two. Those are the same squares you may not drop those pieces onto, in lesson 6 - one rule wearing two hats.