To abruptly understand/realize
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Neutral
当他看到那个工人真的是给警察挂电话，才恍然大悟，原来他们都成了一场骗局的受害者 | Dāng tā kàn dào nàgè gōngrén zhēnde shì gěi jǐngchá guà diànhuà, cái huǎngrándàwù, yuánlái tāmen dōu chéng le yī chǎng piànjú de shòuhàizhě
It was only when he saw the worker actually hang up on the police that he realized they were all victims of the scam
[Source]
听了老师的话，他才恍然大悟 | Tīng le lǎoshī de huà, tā cái huǎngrándàwù
It was only after hearing the laoshi's explanation that it all clicked for him
[Source]
To hit someone (referring to an idea, a realization, etc)
It all clicked (referring to a concept, a realization, etc)
豁然大悟 (huò rán dà wù)
This is an HSK chengyu
《景德传灯录》(Jǐng Dé Chuán Dēng Lù), a work by Sòng (宋, 960-1279) Dynasty writer and monk 释道原 (Shì Dào Yuán)
This chengyu is from a line in the work listed in the "Origin" section above