How std::enable_shared_from_this::shared_from_this

I just cannot understand how std::enable_shared_from_this::shared_from_this returns a shared pinter that shared ownership with existing pointer. In other words you do this:

std::shared_ptr<Foo> getFoo() { return shared_from_this(); }

So when you call getFoo how does exactly it get what is the other shared_ptr to share the ownership with and not to create a separate shared_ptr that owns the same this.

I need to understand this to be able to understand how to create shared_ptr from some object that all increase the same ref count and not initialize separate shared_ptrs.