Istiʿla means elevation. During articulation, the rear part of the tongue rises toward the upper palate, giving the consonant a characteristically heavy quality. It is not identical with itbaq, which involves a more enclosed configuration.
Its seven letters are collected in خُصَّ ضَغْطٍ قِظْ: خ، ص، ض، غ، ط، ق، ظ.
Practice: compare «سَ» with «صَ» and «كَ» with «قَ». Listen for the changed resonance, without treating heaviness as merely louder volume or an exaggerated widening of the mouth. The goal is the authentic elevated quality of each letter.