Al-Ishmam (الْإِشْمَامُ) — is shaping your lips in the shape of a circle without closing them when pronouncing a letter vocalized by dammah when stopping on it. At the same time, we do not pronounce dammah but we shape the lips in the circular form as if we are pronouncing it when making a stop with a sukoon. It is done in order to distinguish between an original sukoon from a non-original one. For example:
Ishmam is made only at the end of a word when stopping. Ishmam is met in the middle of a word only once in the Quran, in surah Yusuf in the word تَأۡمَ۬نَّا.
The root of تَأْمَنُنَا is the first noon with dammah and the second noon is vocalized by fathah. The first noon takes sukoon and then doubles into the second one with Ishmam.