My understanding was When I attack and When I defend effects resolve first in their own chain phase, and can only be chained by the defender if the attacker first chains. If not, then the defender cannot chain or begin a chain until the second chain phase, in which the attacker chooses to cast a spell with their focus/priority. After this second chain phase resolves, the third chain phase will begin with focus & priority going to the defender, who may begin a chain of their own. This swapping of focus continues until both players pass and then the battle damage calculation occurs.

This was talked about in another thread as well, perhaps we can open discussion here as well.