From this article onwards, I'll slowly explain the technique of betaori. For the sake of convenience, let's assume only one person is in tenpai.
Firstly, the most important principle in betaori is:
According to how safe a tile is, discard in order starting with the safest tile
Note, once you decide to betaori, you should discard a tile based on how safe it is, even if it means discarding your mentsu or shuntsu.On an average, betaori happens 30%-40% of the time, therefore a person's betaori skill will directly affect their score.
Today, we'll introduce a few highly safe tiles.
Genbutsu(Degree of safety: SS)
Reusing the previous example:
The tiles here discarded by the player who declared riichi, including 56 pin, 7 wan, 6 sou etc, are known as genbutsu. They're 100% safe to the player who declared riichi. Even if the riichi is waiting for the tiles that the player discarded himself, because of the furiten rule, the player who declared riichi won't be able to win on your tile.
Similarly, any tile discarded after the riichi is genbutsu.
The fourth word tile(Degree of safety: SS)
Of course, if there is any word tile that has been discarded three times, it should be 100% safe to discard unless it's kokushi musou. As kokushi musou has unique discards, and is easily detected by others, added to the fact that the probability of kokushi musou is low, it's degree of safety is on the same level as genbutsu.
Tiles discarded by the player before you in the same turn(Degree of safety: SS)
Due to the furiten rule, tiles discarded in same turn by the player before you is also 100% safe.
Basically, these three are the only absolutely safe tiles, I'll continue to introduce other safe tiles based on the degree of safety.
Exhausted word tiles(Degree of safety: S)
Word tiles that have been discarded twice, even if there's a chance of a hell tanki wait, are relatively safe tile. Firstly, there is little chance of a player who declared riichi to be on a hell tanki wait, because they won't be able to win if the last tile is in the dead wall. Also a word tile waiting on tanki is easy to change, and most would change to a better tenpai before declaring riichi.
(To be continued)