I think the little cloud of smoke appears to divert the viewer's attention away from the fact that the text simply goes from 0 to 100 percent opacity. They may also have used the smoke cloud as a track matte.
You can make smoke with a particle effect or through a noise effect.
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA
Thank you for your answer, that's what I was thinking !
Do you have any idea which key word I can google to find a tutorial to make that smoke cloud ? I don't have the specific terms in english (my after effect is in french so it's hard to find technical terms to google...)