user23747: Chess, Xiangqi (Chinese Chess) and Go all highly value the tempo/initiative.
The Chinese saying "宁失一子,不失一先" literally means 

It is better to lose material/piece, than a tempo/initiative.

(宁=prefer; 失=lose; 一=one; 子=piece (e.g. pawn, rook, knight); 不=not; 先=initiative, tempo, momentum)
I understand the principle/tactic, but I don't know how to express it idiomatically. Is there an English maxim for it?

If there hasn't been an established proverb in English, we may invent some.
For example,
@T.J.Crowder:
"Damn the material, full speed ahead!"
@Remellion:
"Speed over greed" 
Pros: rhythm.
Cons: The saying emphasizes to accept your own pieces' loss; while "greed" is about taking your opponent's pieces.

user20715: Looking at this slightly obliquely:

A stitch in time saves nine

In other words, spending a small amount of effort to do a small thing (e.g. sew a stitch into a piece of fabric that has started to tear or fray) before it gets worse (i.e. "in time") prevents you from having to spend far more effort later ("nine" - literally meaning needing to do nine times as many stitches later)
On a similar note - but, I think, far less related to your own idiom - there is

More haste, less speed

which means "if you try to rush something now, and have to redo it properly later, then it will take more time in total"

user23814: When I used to play chess, I would say something like "I sacrificed pieces to gain/maintain tempo".

user15223: Not exactly equivalent to the Chinese saying, but sacrificing material for initiative is also known as "playing like Tal". Tal was well-known as the greatest attacking player the world had ever seen, who's produced many magical, sacrificial combinations.

user23800: One fiery saying that might work is this one.
"Damn the torpedoes, full speed ahead!"

user23809: "You can't make an omelette without breaking eggs"
This is a good English idiom for accepting losses/sacrifices to achieve your goal. It doesn't express anything about keeping up the tempo/pressure though. 

user23804: It sounds like you want to keep up the pressure. You might be taking a loss of material, but a coherent, rapid, intimidating overall strategy maintains your tempo.

user15989: Excellent question. Translation is always a tricky business.
There isn't an established chess saying, although Patrick McElhaney's answer "Time is money" comes closest in meaning, and is an idiom in English.
So I would simply invent another phrase with the same meaning, if you want to convey the idea and keep the catchiness that it has in Mandarin. I propose "Speed over greed", which rhymes and is relatively self-explanatory.

user10570: I am not aware of an idiomatic way of saying it. However, I would express it as "seizing the moment".
For example, there is a book by Ivan Sokolov, "Sacrifice and Initiative in Chess: Seize the Moment to Get the Advantage," which I think talks exactly about the importance of the initiative vs material that you mean in your question.

user16949: Time is money.
It’s an idiom not typically used in the context of chess, but I think the meaning would be clear in context to a native English speaker. You’re losing “money” (a piece) in order to gain “time” (a tempo). You expect to gain an advantage because the latter is more valuable. 
In more formal economics lingo, there’s opportunity cost. For example, your opponent offers a trade by taking a piece that is guarded. You choose not to take the piece back because the opportunity cost of losing a tempo (or two) is higher than the cost of being down a piece. 

user20260: I might not understand the Chinese idiom correctly, but to me it sounds like--
"Lose the battle but win the war"
--which has the meaning of, it is better to sacrifice some small thing for the bigger picture (to me, that sounds like the crux of the Chinese idiom you shared). 

23747: <p>Chess, Xiangqi (Chinese Chess) and Go all highly value the tempo/initiative.</p>

<p>The Chinese saying "宁失一子,不失一先" literally means </p>

<blockquote>
  <p>It is better to lose material/piece, than a tempo/initiative.</p>
</blockquote>

<p>(宁=prefer; 失=lose; 一=one; 子=piece (e.g. pawn, rook, knight); 不=not; 先=initiative, tempo, momentum)</p>

<p>I understand the principle/tactic, but I don't know how to express it idiomatically. Is there an English maxim for it?</p>

<hr>

<p>If there hasn't been an established proverb in English, we may invent some.</p>

<p>For example,</p>

<p><strong>@T.J.Crowder</strong>:</p>

<p>"<strong>Damn the material, full speed ahead!</strong>"</p>

<p><strong>@Remellion</strong>:</p>

<p>"<strong>Speed over greed</strong>" </p>

<p>Pros: rhythm.</p>

<p>Cons: The saying emphasizes to accept your own pieces' loss; while "greed" is about taking your opponent's pieces.</p>
 20715: <p>Looking at this slightly obliquely:</p>

<blockquote>
  <p>A stitch in time saves nine</p>
</blockquote>

<p>In other words, spending a small amount of effort to do a small thing (e.g. sew a stitch into a piece of fabric that has started to tear or fray) before it gets worse (i.e. "in time") prevents you from having to spend far more effort later ("nine" - literally meaning needing to do nine times as many stitches later)</p>

<p>On a similar note - but, I think, far less related to your own idiom - there is</p>

<blockquote>
  <p>More haste, less speed</p>
</blockquote>

<p>which means <em>"if you try to rush something now, and have to redo it properly later, then it will take more time in total"</em></p>
 23814: <p>When I used to play chess, I would say something like "I sacrificed pieces to gain/maintain tempo".</p>
 15223: <p>Not exactly equivalent to the Chinese saying, but sacrificing material for initiative is also known as "<a href="https://www.bing.com/search?q=play%20like%20tal%20chess" rel="nofollow noreferrer">playing like Tal</a>". Tal was well-known as the greatest attacking player the world had ever seen, who's produced many magical, sacrificial combinations.</p>
 23800: <p>One fiery saying that might work is this one.</p>

<p>"Damn the torpedoes, full speed ahead!"</p>
 23809: <p>"You can't make an omelette without breaking eggs"</p>

<p>This is a good English idiom for accepting losses/sacrifices to achieve your goal. It doesn't express anything about keeping up the tempo/pressure though. </p>
 23804: <p>It sounds like you want to <strong>keep up the pressure</strong>. You might be taking a loss of material, but a coherent, rapid, intimidating overall strategy maintains your tempo.</p>
 15989: <p>Excellent question. Translation is always a tricky business.</p>

<p>There isn't an established chess saying, although Patrick McElhaney's answer "Time is money" comes closest in meaning, and is an idiom in English.</p>

<p>So I would simply invent another phrase with the same meaning, if you want to convey the idea <em>and</em> keep the catchiness that it has in Mandarin. I propose <strong>"Speed over greed"</strong>, which rhymes and is relatively self-explanatory.</p>
 10570: <p>I am not aware of an idiomatic way of saying it. However, I would express it as "<em>seizing the moment</em>".</p>

<p>For example, there is a book by <a href="https://www.newinchess.com/en_US/catalog/product/view/_ignore_category/1/id/601/s/sacrifice-and-initiative-in-chess/" rel="noreferrer">Ivan Sokolov, "Sacrifice and Initiative in Chess: Seize the Moment to Get the Advantage,"</a> which I think talks exactly about the importance of the initiative vs material that you mean in your question.</p>
 16949: <p><em>Time is money.</em></p>

<p>It’s an idiom not typically used in the context of chess, but I think the meaning would be clear in context to a native English speaker. You’re losing “money” (a piece) in order to gain “time” (a tempo). You expect to gain an advantage because the latter is more valuable. </p>

<p>In more formal economics lingo, there’s <em>opportunity cost</em>. For example, your opponent offers a trade by taking a piece that is guarded. You choose not to take the piece back because the opportunity cost of losing a tempo (or two) is higher than the cost of being down a piece. </p>
 20260: <p>I might not understand the Chinese idiom correctly, but to me it sounds like--</p>

<p>"<a href="https://www.macmillandictionary.com/us/dictionary/american/lose-the-battle-but-win-the-war" rel="noreferrer">Lose the battle but win the war</a>"</p>

<p>--which has the meaning of, it is better to sacrifice some small thing for the bigger picture (to me, that sounds like the crux of the Chinese idiom you shared). </p>
