How does Retreat function and when do you do it?

Also, when does 'play only during your build step' apply?

 

I believe its: Dark side builds, light side builds, dark side retreats, light side retreats. But as I understand it you can unretreat during your build step.

 

Yes, you unretreat your units during your build step, and you may unretreat as many, or none, of your units as you like.

The cost of retreating a unit in an arena into the build zone is to tap it. You cannot retreat a tapped unit (ANH rulebook page 17), and you cannot use a retreated unit's abilities unless that ability can prevent damage (ANH rulebook page 17 and 38), or it is qualified as a Reserves ability (new in Jedi Guardians). Also remember that when your units untap at the start of a new turn, this includes your retreated units.

When the ESB rulebook is released (November, kiddies!), it will make a rules addendum/modification/clarification that you may deploy a unit immediately into the build zone (retreated), that way you don't have to deploy into an arena, then tap it to retreat it. Since this particular ruling was stated on the WotC site as an answer to a "Question of the Week", it should apply now.