Atozuke (1)
For example, suppose you have a hand like Figure 1.
Figure 1: 
Here, the normal and most common way to open is to start from , which gives Figure 2.
Figure 2: 
By contrast, if the player on your left discards 
and you call there,
you get the shape known as yakuhai-back, shown in Figure 3.
Figure 3: 
Now compare Figure 2 and Figure 3.
In terms of how easy they are to win, there is not much difference between them.
It is true that in Figure 3 you cannot win unless comes out.
And yes, there are cases where the other players are each holding two copies of it.
That is the so-called mochimochi case.
But in practice, that possibility is not nearly common enough to reject this line from the start.
Figure 3 also keeps a backup route.
For example, if you draw 
, you can move straight into Tanyao.
The remaining shape is also good, so winning is not nearly as difficult as it may seem.
Now think about what happens if someone riichi's against you.
Figure 2 is the one that becomes uncomfortable.
With Figure 3, you can cut the pair of and survive for two turns.
Many people dislike atozuke, but depending on the hand, opening with a back-up yakuhai is absolutely a valid option.
You should understand that:
atozuke is a call pattern with very strong defensive value.
Because when things go wrong, it still lets you throw the yakuhai pair away.
Atozuke Is More Common in Red-Five Mahjong
In fast red-five mahjong, there are actually quite a lot of situations where you open from an atozuke shape.
With a hand only around the level of Figure 1, you do not need to force atozuke.
But if it becomes something like this:
Dora
Now you already have a 3900-point hand if you call.
In that case, passing on from the player on your left is simply too soft.
It means your sense of speed is a little weak.
The kinds of hands where you should open even with atozuke are, in the end, hands with two or more dora.
Those hands can already reach a high value without needing to stay closed and riichi.
If the hand has dora, speed should come first.
Figure 4
Figure 4: 
Dora
If you get a hand like Figure 4 in a real game,
you should call on every tile you are allowed to call.
Do Not Break Your Own Atozuke Entrance in a Chance Hand
Figure 5
Figure 5: 
Dora
Suppose you are East dealer in East 1 with the hand in Figure 5.
How should you approach it?
This is already a guaranteed dealer mangan, so an all-call approach is not completely impossible.
But that style is more like the kind of play that crushes weaker opponents.
Once the level goes up, it stops working well.
Basically, I do not think you should chi 
or .
If this is a chance hand that you really want to win,
you should try not to break apart your yakuhai-back entrance too early.
Otherwise, players may clamp down not only on ,
but even on as well.
Pon 
Chi 
After showing a call pattern like this,
only a beginner is likely to throw your dora yakuhai or double East into you.
Figure 4 is different.
If you start calling from the pinzu or souzu side there, the hand can still look like Tanyao from the outside.
That is why in Figure 4 you can call from almost anywhere.
Figure 5 is the opposite.
Do not break your own atozuke entrance too early.
Original Japanese page: http://beginners.biz/naki/naki11.html