Īghāl (الإيغال) is an additional benefit placed at the end of a verse-line after the sense is already substantially complete. The ending sharpens the depiction or intensifies the intended effect.
The source gives the poetic line:
«وَإِنْ صَخْرًا لَتَأْتَمُّ الْهُدَاةُ بِهِ، كَأَنَّهُ عَلَمٌ فِي رَأْسِهِ نَارُ»
Ṣakhr is such a guide that even guides follow him; he is like a conspicuous mountain landmark with a fire atop it.
The ʿalam (عَلَم) is already conspicuous, but “with fire on its summit” makes the simile even stronger. That further effect marks the rhetorical addition.