Apparently something new female students are doing – you can see and hear some of it in the infamous Yale video...
For the “mass audience”, if someone says something you don’t like, you might boo, or shout “no”. For the “enlightened”, snapping fingers in front of their face is the way to express it (maybe derived from upper class finger snapping at servant class). Mass finger snapping is the way of engaging the group-think of an “enlightened” group of people and signaling their disrespect of the speaker.