Defence theory(13): Detailed explanation of each type of melded sets(Part 2)

Source: http://blog.jpmahjong.net/read.php/558.htm

The melded sets mentioned in the previous article, all belong to "sakidzuke katachi". What it means is that the melded sets are related to yaku. Sometimes, the first and second melded sets of the opponent may not be related to yaku. The aim of calling those tiles is to get rid of the bad shape of the hand. This type of attack is referred to as "atodzuke katachi", for example:

[First image in http://blog.jpmahjong.net/read.php/558.htm]

or

[Second image in http://blog.jpmahjong.net/read.php/558.htm]

Normally, "atodzuke katachi" melded sets have a higher value than "sakidzuke katachi", but as the possibility of the hand is heavily restricted, it's not hard to find out what tiles are dangerous. This article will discuss how to defend against this type of attack.

These two are commonly seen atodzuke yaku:
1) Yakuhai
2) Sanshoku, Itsuu, Honitsu

Among them, yakuhai has the highest probability of appearing. So once melded sets appear, the first thing you should check for are live yakuhai tiles. (capable of making a koutsu.) If you have such tiles, there are two choices you can take:
The first is to discard it immediately, hoping that the opponent is not in tenpai.
The second is to keep it until the end of the hand.

If the opponent starts to discard the tiles he draw, then the chances of being near tenpai is high. The live yakuhai tiles now are dangerous tiles, and should not be discarded.
If you've made sure that yakuhai is not possible, then the only possibility now is sanshoku and itsuu. Most of the time, you can rely on the discard pond to estimate the tiles related to yaku, and avoid discarding them. As for the value of the hand, you can estimate using the amount of dora you haven't seen. If the dora is related to yaku(for example the dora is yaochuhai and the hand is hon chanta), you need to be more careful.

Using a simple example
[Third image in http://blog.jpmahjong.net/read.php/558.htm]

Kamicha calls 2 pin. You can tell that it's not a honitsu by looking at the discards. You can see four 4 sou and 9 pin, therefore the chances of itsuu and sanshoku is gone. The only possibilities left are hon chanta and yakuhai. From the discards, the possibility of the former is rather high.
The yakuhai that has not appeared yet are haku and hatsu, and since 9 wan is the dora, these three tiles are highly dangerous. Secondly, the tiles that can form shuntsu of 123 and 789, like 78 sou, 2378 wan etc are not safe.

From the standpoint of the top player, these tiles must not be discarded.