や否いなや

や否や marks that something happens immediately after another action, with a sense of the first event being instantly followed by the second. It is formal, literary, and often used in written narration or descriptive prose. It conveys a strong immediacy, similar to 'no sooner than' or 'the moment...'. It is typically attached to a verb in the dictionary form (non-past) and used to describe a rapid succession of events.