How does auto-login work?
You can enable auto-login in the connector module.
The *Auto Login Restricted by IP address* will require that the customer clicking on the link is at the same IP address that they were at when they placed the order or abandoned the cart in question that triggered the email.
Many times people are on dynamic IP addresses, so your hit rate for successful auto-logins will be lower with that enabled - but it's safer from a security perspective.
The auto-login happens client-side after the first page load. So for example if they click through to a product from an email and then add that product to cart, by the time they add to cart they're already logged in.