One rhetorical function of ellipsis is to test whether the audience follows the discourse attentively. When a contextual cue is available but requires some reflection, the speaker leaves the subject unspoken and allows the listener to recover it.
أو اختبار تنبه السامع عند القرينة
Or to test the listener's alertness when a contextual indication exists.
Suppose a teacher has been discussing a particular student and continues: «مُجْتَهِدٌ وَلٰكِنَّهُ مُتَسَرِّعٌ» — “[He is] diligent but hasty.” The earlier context supplies the subject. The purpose is not absolute secrecy but engaged comprehension.
If the cue is so obscure that ordinary listeners cannot identify the student, the omission ceases to be an effective exercise in alertness and becomes misleading ambiguity.