You are talking about a diaeresis [1] For "rien" in particular, I'd say speakers can do that diaeresis as a sort of emphasis. For instance "Tu n'as rien fait!

You are talking about a diaeresis [1] For "rien" in particular, I'd say speakers can do that diaeresis as a sort of emphasis. For instance "Tu n'as rien fait! Rien! ("you did nothing! Nothing!"). The first "rien" can be said as a single-syllable world, while the second one can be pronounced as a two-syllables. It also may be pronounced slightly differently in various places in France (regional accents). https://en.wikipedia.org/wiki/Diaeresis_%28prosody%29 https://en.wikipedia.org/wiki/Diaeresis_%28prosody%29