In ijaz al-hadhf, a word, clause, or several clauses are left out but recoverable from the expression or its context. A qarinah (clue) guides the listener to what is understood.
Grammar, established usage, and the immediate context can identify an omitted word. The sequence of events may reveal an omitted clause; narrative compression can imply several intervening actions.
The critical distinction is between purposeful omission and obscurity. Without a reliable way to recover the intended sense, omission is not successful brevity.