Ṣadr identifies the earlier position to which a final expression refers, while ʿajz denotes the end of the utterance or verse. The term does not require the first related word always to be the first word of a line.
In verse, the earlier word may appear at the start, middle, or end of the first hemistich, or at the beginning of the second. The closing expression still completes the pattern.
Its effectiveness depends on meaningful closure, not forced or redundant duplication.