I understand that French literature uses past historic. But I also see some imperfect used mixed with past historic in literature. Without hopefully violating copyright with the below sample, here is an example from Harry Potter official translation :
Elle donna un affectueux coup de bec à Harry tandis qu'il lui enlevait son fardeau, puis elle traversa la pièce d'un coup d'aile pour rejoindre Errol.
Here, "donna" is historic and "enlevait" is imperfect. What is the difference in meanings of these past tenses when they are mixed ?