The idea behind this is to have the ASG act as a load balancer. So if one of your "real servers" goes down, the ASG only forwards requests to the remaining "real servers". It's you job to have the same content on that two "real servers" ;-)
AFAIK it's a pretty basic round robin, but i'll ask the developers, maybe they have some links to the documentation where this is explained in depth. Stay tuned ;-)
