Istitalah means extension. This quality belongs only to ض (dad). Its pronunciation involves a particular extension of articulation along the relevant side of the tongue, distinguishing it from neighboring consonantal sounds.
Dad is not simply a heavy د or an interchangeable ظ. Along with istiʿla and itbaq, it has its own istitalah. Tongue placement, the area of contact, and the direction of sound must be coordinated.
Practice: pronounce «أَضْ» in front of a qualified teacher and compare it with «أَظْ» and «أَدْ». This challenging consonant particularly benefits from direct acoustic and articulatory correction.