How to declare
On your turn, with a closed ready hand, say "riichi", place your discard sideways, and put a 1,000-point stick in the centre of the table. Most rules also require at least a few tiles to remain in the wall. From then on you may not change your hand: you must discard every drawn tile that does not win, except for certain closed kongs that leave your waits unchanged.
What you gain
Riichi itself is a yaku worth one han, so it lets any closed hand win. Winning within one go-around of the declaration, uninterrupted, earns ippatsu, another han. After winning you also check ura-dora under the dora indicators for more value. The deposit goes to the next winner.
What you lose
Your hand is now fixed and your intention public. Opponents will discard safe tiles against you, and if someone else declares riichi you cannot fold. A riichi that fails to win costs you the deposit.
When to declare
A good general rule is to declare riichi when your wait is reasonable and your hand has no other yaku, or when the extra han makes a significant difference. With a strong open hand or a terrible wait, experienced players sometimes stay quiet (dama) instead.
