«وهو أن يجيء قبل حرف الروي»
It is when an additional correspondence appears before the poetic rhyme letter. A comparable feature can occur before the relevant part of a prose ending, provided the addition is not already required for ordinary rhyme or sajʿ.
The device also bears names such as iltizām, taḍmīn, tashdīd and iʿnāt. They express the common idea of voluntarily imposing a greater verbal constraint than the basic ornament demands.
First identify the required rhyme or prose ending; only then ask which additional preceding sound or sounds have been voluntarily maintained.