The present marks the action as continuing (not completed), the aorist marks the action as simply occurring (completed). The present usually sets forth an action contemporaneous with that of the leading verb; the aorist, an action antecedent to that of the leading verb.
a. The present may denote time antecedent when the verb has no aorist, and in the case of some other verbs: Thus, (ὁ πόλεμος) δς λυ_πήσει ἕκαστον, ἐπειδὰν παρῇ the war
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].