わけ in this sentence?

Question

どうやったら、この局面でこの指し手になるわけだ？

I'm not sure what this わけ could be. The fact that this is a question bothers me a bit.

Answer 1

jisho.org here defines that 訳｛わけ｝ is as follows:

訳｛わけ｝

noun
conclusion from reasoning, judgement or calculation based on something read or >heard; reason; cause; meaning; circumstances; situation​

My attempt is as follows:

どうやったら、この局面でこの指し手になるわけだ？
How could you choose this move in the state of the game?

My attempt means that I could not understand the reason why you chose/decided this move in the state of the game.

---

Bonus

(0) どうやったら、この局面でこの指し手になるわけだ？

I think that the above-mentioned Japanese sentence of (0) posted by the questioner is not necessarily good, is different from the ordinary way of expressions, or extremely speaking it is a bad sentence, so I will add explanations to (0) and introduce some correct Japanese sentences for those who are studying Japanese.

1. The sentence of (0) is one said in the following situation.
   The situation is a scene of a game of 将棋{しょうぎ} Shogi, 囲碁｛いご｝ Igo or チェス Chess. In the scene of the game, one of the opponents moved a certain piece. However, the move was considerably out of expectation or unpredictable from the established tactic or usual method generally considered in the situation of the game. In this awesome situation, the phrase of (0) was uttered by the opponent of the game or by a third party watching this game with surprising and/or confused emotions.

2. Usually "どうやったら" could be used in a phrase like "どうやったらこの機械を操作できるか？ How can I manipulate this machine?" or "どうやったらこの岩を動かすことができるか？ How can I move this rock?"
   In general this phrase is an expression related to concrete behavior so that it can be paraphrased like "どのようにしたら（何かができる） how to do/behave (to be able to do something)." or "どういう手段を使ったら what kind of means to use "to be able to do something". However, in the sentence of (0), this phrase is used as an expression related to mental or intelligent activity, and I think that it can be paraphrased as "どのように考えたら how to think (to do something)" or "どういう理屈があったら what kind of reasons could lead (to do something)".

3. "局面｛きょくめん｝" is generally used as "situation aspect of an affair; situation", and it is a word came from a game such as Shogi, Igo or Chess meaning "position in a game of Shogi, Igo or Chess; state of the game", so as for the sentence of (0), it is used as the meaning of its origin.

4. "指{さ}し手｛て｝" means a certain move of a piece (in Shogi, Igo, Chess, etc.). Strictly speaking, "move" could be used safely for Shogi and Chess because they are games where players move pre-placed pieces, but as for Igo, they additionally place white or black stones, which are equivalent to pieces in Shogi or Chess, on the surface of a board, so "指{さ}し手｛て｝" in Igo means "placing a stone at a certain place on the surface of the game board" instead of a "move of a piece".

I know that an interrogative use of "訳｛わけ｝だ？" is rare in general Japanese. Therefore I searched for sentences including "訳｛わけ｝だ" in a famous corpus named "[少納言]｛Shounagon｝", and the result is as follows, where sentences in which 訳｛わけ｝だ is used with the same meaning are excluded from the result.

① ベガの意識の中では、ゲージはトイレだった事になる。その「ゲージ」で寝かして居た訳だ！ ② 不吉なイメージなのでこの曲には不適切な訳だ！と言っていました。
③ これで、マリオワールドの世界を一周したことになる訳だ！
④ マイク・シーガーもお爺ちゃんになっちゃいました。私も歳を取る訳だ ・・・（＾ω＾）
⑤ 姿が見えなくてもおかしくないと思っている訳だ
⑥ その突堤の先端は、十三号艇の碇泊している箇所にずっと近付ける訳だ。
⑦ 大寺さんにはさっぱり判らない。―どう云う訳だろう？　と大寺さんは細君に云った。
⑧ サウナに入り、からっからっに乾いたページをめくりながら、旅の汗を流せ、とでもいう訳だろうか。

Among more than 290 example sentences, ⑦ and ⑧ are only two interrogative sentences. Therefore, it seems that the ending part of the sentence posted by the questioner should be like "～この指し手になるわけだろうか？" as correct Japanese.

Based on the above analysis, if the sentence of (0) is rewritten to correct Japanese that is easy to understand, it becomes like the following (1) to (4).

Original:

(0) どうやったら、この局面でこの指し手になるわけだ？

My attempts:

(1) どのように考えたら、この局面でこの指し手になるわけだろうか？
(2) どのように考えたら、この局面でこの指し手が正当であるという理屈になるのだろうか？
(3) どういう理由があって、この局面でこの指し手になるのだろうか？
(4) どういう理屈/理由に基づいて、この局面でこの指し手になるのだろうか？