Qalqalah
Qalqalah — the bouncing letters قَلْقَلَة
A subtle "echo" on the five letters قطب جد (ق ط ب ج د) when they carry sukoon.
When the five qalqalah letters (ق، ط، ب، ج، د — mnemonic: قطب جد) carry a sukoon, the reader gives them a small bounce — a subtle vocal "echo" without adding a real vowel. There are three degrees: qalqalah sughra (small) on a sukoon mid-word; qalqalah kubra (large) on a sukoon at the very end when stopping; and qalqalah akbar (greatest) when stopping on a letter with shadda.
Quran examples
يَطْمَعُ
Sughra — qalqalah on ط mid-word. · 74:15
وَتَبَّ
Akbar — stopping on the shadda of ba in tabba. · 111:1