As discussed in our だに＋しない grammar point, だに is an adverbial particle with a function similar to さえ and すら ('even'), though it is primarily found in older or more literary Japanese. It is used to present (A) as the bare minimum or a very slight action, suggesting that anything beyond (A) would be difficult to imagine, endure, or comprehend. As a result, it is often translated as 'just (A)' or 'merely (A)'.

This particle is attached directly to the dictionary form of a verb.

• あのホラー映画(えいが)の音(おと)を聞(き)くだに、怖(こわ)くて眠(ねむ)れなくなる。
  Merely hearing the sound from that horror movie makes me too scared to sleep.
• 山頂(さんちょう)からの景色(けしき)を想像(そうぞう)するだに、胸(むね)が高鳴(たかな)る。
  My heart pounds with excitement just imagining the view from the mountaintop.
• 昨年(さくねん)の大(おお)きな失敗(しっぱい)を思(おも)い返(かえ)すだに、顔(かお)が赤(あか)くなる。
  My face turns red just recalling my big mistake from last year.

だに is a formal expression primarily encountered in written works rather than everyday conversation. That being said, it is not unusual to hear it in anime or manga during particularly dramatic or emotional scenes.

Since (A) represents the lowest possible threshold, it is often a verb related to thought or perception, creating a contrast between an internal state and external reality. For instance, with 想像(そうぞう)するだに, 'imagination' is presented as the bare minimum, which stands in stark opposition to what might happen 'in real life'.