Post History

This is simply backstory, so the rules of backstory apply. Backstory should only be given when the reader **wants** to know the backstory. Backstory slows the forward momentum of the plot, so it should only be given when the lack of backstory would make further forward progress of the plot impossible. 

This is not at all about the technique for introducing the backstory. It is entirely about timing. The principle here is the same as the principle for coincidence: you can use any coincidence you like to get your hero into trouble, but not to get them out. Similarly, you can use any contrivance you like to get welcome backstory into the novel, but no contrivance at all will work to get unwelcome backstory into the novel. 

So figure out what you can do to make the reader demand to know the backstory, which generally means the point where they feel they can't go on in the story without knowing the backstory, and then give it by any means you like. 

Alternatively, work the backstory into a scene in which something else is going on at the same time as the backstory is being given. For instance, use dialogue that crackles with conflict or that reveals character that uses backstory as the material for that conflict or to reveal that character. That way the backstory comes out in the course of the drama, not as an interruption of it. (But note that this means as part of the substance of the drama, not interspersed with it.)