Push, pull mechanism observer pattern

At the moment i'm studying design patterns and i've came to a part where i'm confused whether the observer pattern makes use of the push mechanism or does it makes use of the pull mechanism ?

I've read different implementations of this and can't realy establish which one is correct.

Also i'd like to know three straight forward advantages of the push model towards the pull model. I guess one of them is that the push model is less coupled then the pull model ?