Jan 07 - Difference between Preterito Indefinido and Imperfecto

One of my Flashcards says this:

"He came, when i left"-venía cuando me fui (original German is "Er kam, als ich ging")

I don't understand why you would use Indefinido first, and then Imperfecto?! That doesn't make any sense, can someone explain ? Or are those Flashcards just incorrect?

I found a Source explaining the difference between both Tenses (http://www.learn-spanish-online.de/grammar/chapter7_compound_tenses/7_4_use_perfecto_indefinido_imperfecto.htm)

1. There are two actions in the past that happen at the same time.

We have two actions here, so you would use the Imperfecto only on the first verb? Is that the Rule or? Someone explain