With the hand on the bottom, it's supposedly better to discard rather than , which may seem rather counter intuitive at first. Fukuchi was kind enough to write up a brief article on his blog explaining the logic more fully, so I've decided to translate it here.
I think this was from a foreigner.
This is a common WWYD mistake taken from Uzaku's book.
This WWYD came up in discussion in Osamuko's Discord chat. I understand the reasoning behind discarding 2s but I'm not entirely convinced.
２５ｍは５７ｓより先に来るでしょうし、一番目の前のテンパイは薄いカン５ｓになります。手配の形を広げる時間がなければ打８ｓでも良いのでは？— Muller (@Muller_Mahjong) September 10, 2016
You're more likely to get before , so your immediate tenpai would be a weak kanchan. If you don't have the time to widen your hand shape, isn't it fine to discard ?
In this problem -- dora: there's not really an immediate difference between discarding and discarding .
(The dora is , so let's forget about making a pair from the manzu.)
Depending on the situation, you might sometimes want to fix your souzu shape as a pair plus a kanchan.
But if you think about this problem one turn into the future rather than just in the here and now, it changes considerably.
If you discard here and draw any of , , , , , you get a nice, continuous shape.
Think about the possibility of making a continuous shape.
Even with a hand like this that seems unlikely to extend -- dora:
If you discard , you could draw .
As long as there are no other circumstances such as the dora being or sanshoku being possible, it's better if you don't solidify your hand shape.
The key is to play flexibly.
When you think about it it's a rather simple lesson, and it may seem like stating the obvious, but many people (myself included) still often fall into the trap of fixing the shape of their hand. The two key takeaways here are this:
Don't make decisions simply based on the immediate circumstances, but consider the possibilities of future developments. By the way, this also applies when deciding whether you should riichi immediately or try to improve your wait.
At iishanten or ryanshanten it's easier to form pairs than to complete a mentsu. If your hand shape can easily widen, it becomes even easier to form pairs.