Shadda causes the letter it comes upon to be read in double tone, the first sound with a sukoon (stopping) and the second with a Harakat that is also comes with a Shadda (Damma, Kasrah, Fatha).

For example: when we type the word; اِنّ it actually spelled as this اِنْ نَ and it is pronounced as Innominate example when we say Allah, in Arabic form the letter L has ‘Shadda and Fatha’ at the same time so we read in this manner.