«وَإِذَا أَوْجَزَ أَعْجَزَ»
When he is concise, he produces a feat difficult to rival.
Awjaza means to express much with few words; aʿjaza suggests leaving others unable to match the achievement. A small phonetic change brings a meaningful shift.
Read together with ashaba / adh-haba, the pair describes a writer at ease with both extremes of length. The ideal is not a fixed number of words but the right degree of development for a particular meaning and occasion.
The remaining short, parallel clauses also praise impromptu invention: the unnamed writer is said to create effective language when challenged, not merely repeat stored models.