How to generate activation email

I have been using Membership for a while now. It is currently set up for free subscriptions only.
The way it currently works once the person selects their subscription they then go to a page where they enter their details, once they hit submit they automatically become activated members. Is there a way to configure so they are sent an activation email first, I can't seem to work out how to do this.
I do have allow incomplete registration checked at the moment.