Isolation during sessions

I am a bit confused by the isolation during sessions, in my understanding during a session i always got a snapshot of the data, meaning that i cant see modifications to collections while in that session, but using the java drive I run up with this method ClientSession.getOptions().isSnapshot(); what made me believe that somehow you could see changes happening inside a session. Can someone explain me kinda in depth how this works? I already read the documentation but could not make sense out of it.